Added ability to return the /help endpoint as a JSON object.
Added ability to return the /help endpoint as a JSON object.

Previously, the /help endpoint could only be returned as either pure
markdown or html.

This commit introduces a query parameter: format=json to allow the
help endpoint to return a JSON object containing the help information
for all other endpoints.

Review: https://reviews.apache.org/r/43217/

reneploetz/mesos,verizonlabs/mesos,Gilbert88/mesos,neilconway/mesos,chhsia0/mesos,yuquanshan/customizedMesos,abudnik/mesos,craimbert/mesos,asamerh4/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,andschwa/mesos,asamerh4/mesos,Gilbert88/mesos,andschwa/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,andschwa/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,gsantovena/mesos,craimbert/mesos,gsantovena/mesos,andschwa/mesos,abudnik/mesos,verizonlabs/mesos,dforsyth/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,neilconway/mesos,verizonlabs/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,Gilbert88/mesos,chhsia0/mesos,asamerh4/mesos,neilconway/mesos,neilconway/mesos,jpeach/mesos,yuquanshan/customizedMesos,zmalik/mesos,gsantovena/mesos,kaysoky/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,jpeach/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,reneploetz/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,zmalik/mesos,zmalik/mesos,reneploetz/mesos,reneploetz/mesos,verizonlabs/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,craimbert/mesos,chhsia0/mesos,neilconway/mesos,kaysoky/mesos,craimbert/mesos,Gilbert88/mesos,zmalik/mesos,verizonlabs/mesos,jpeach/mesos,jpeach/mesos,asamerh4/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,jpeach/mesos,gsantovena/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,verizonlabs/mesos,craimbert/mesos,andschwa/mesos,asamerh4/mesos,craimbert/mesos,neilconway/mesos,dforsyth/mesos,yuquanshan/customizedMesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,zmalik/mesos,chhsia0/mesos,chhsia0/mesos,andschwa/mesos,asamerh4/mesos,asamerh4/mesos,zmalik/mesos,craimbert/mesos,dforsyth/mesos,abudnik/mesos,zmalik/mesos