Mark android-support library as optional.
Mark android-support library as optional.

Projects that don't use the support library shouldn't have to install it
for robolectric to work. It should only be needed for projects that use
the support library shadows (and will have the support library available).

Closes #1087.

zhongyu05/robolectric,erichaugh/robolectric,mag/robolectric,wyvx/robolectric,cesar1000/robolectric,pivotal-oscar/robolectric,1zaman/robolectric,diegotori/robolectric,eric-kansas/robolectric,trevorrjohn/robolectric,macklinu/robolectric,WonderCsabo/robolectric,gruszczy/robolectric,gruszczy/robolectric,eric-kansas/robolectric,paulpv/robolectric,charlesmunger/robolectric,gb112211/robolectric,gabrielduque/robolectric,yuzhong-google/robolectric,gruszczy/robolectric,amarts/robolectric,VikingDen/robolectric,kriegfrj/robolectric,tmrudick/robolectric,cc12703/robolectric,toluju/robolectric,ocadotechnology/robolectric,lexs/robolectric,1zaman/robolectric,pivotal-oscar/robolectric,fiower/robolectric,VikingDen/robolectric,davidsun/robolectric,WonderCsabo/robolectric,rburgst/robolectric,tyronen/robolectric,jingle1267/robolectric,rongou/robolectric,lexs/robolectric,svenji/robolectric,rburgst/robolectric,gabrielduque/robolectric,kriegfrj/robolectric,karlicoss/robolectric,davidsun/robolectric,cc12703/robolectric,WonderCsabo/robolectric,holmari/robolectric,hgl888/robolectric,VikingDen/robolectric,tmrudick/robolectric,toluju/robolectric,svenji/robolectric,tec27/robolectric,karlicoss/robolectric,macklinu/robolectric,tec27/robolectric,tmrudick/robolectric,ocadotechnology/robolectric,karlicoss/robolectric,1zaman/robolectric,tyronen/robolectric,yuzhong-google/robolectric,ChengCorp/robolectric,zbsz/robolectric,svenji/robolectric,zbsz/robolectric,plackemacher/robolectric,jongerrish/robolectric,tjohn/robolectric,hgl888/robolectric,jingle1267/robolectric,paulpv/robolectric,pivotal-oscar/robolectric,wyvx/robolectric,tjohn/robolectric,amarts/robolectric,rburgst/robolectric,zhongyu05/robolectric,diegotori/robolectric,plackemacher/robolectric,charlesmunger/robolectric,kriegfrj/robolectric,erichaugh/robolectric,toluju/robolectric,jingle1267/robolectric,gabrielduque/robolectric,erichaugh/robolectric,trevorrjohn/robolectric,holmari/robolectric,hgl888/robolectric,lexs/robolectric,spotify/robolectric,jongerrish/robolectric,cc12703/robolectric,trevorrjohn/robolectric,tyronen/robolectric,tuenti/robolectric,cesar1000/robolectric,cesar1000/robolectric,fiower/robolectric,tjohn/robolectric,spotify/robolectric,holmari/robolectric,zhongyu05/robolectric,gb112211/robolectric,rongou/robolectric,tec27/robolectric,jongerrish/robolectric,ocadotechnology/robolectric,charlesmunger/robolectric,gb112211/robolectric,ChengCorp/robolectric,rongou/robolectric,spotify/robolectric,mag/robolectric,zbsz/robolectric,davidsun/robolectric,plackemacher/robolectric,eric-kansas/robolectric,wyvx/robolectric,tuenti/robolectric,macklinu/robolectric,diegotori/robolectric,yuzhong-google/robolectric,amarts/robolectric,ChengCorp/robolectric,paulpv/robolectric,fiower/robolectric,mag/robolectric,tuenti/robolectric,jongerrish/robolectric