guard against server only options being configured at location scope
guard against server only options being configured at location scope

guard against server only options being configured at location scope this
introduces a duplicated list of server-level-only options from mod_instaweb.cc,
which is a short term solution and should be removed later on

Fixes #99.

wanrui/ngx_pagespeed,0x0mar/ngx_pagespeed,pagespeed/ngx_pagespeed,pagespeed/ngx_pagespeed,wanrui/ngx_pagespeed,0x0mar/ngx_pagespeed,digideskio/ngx_pagespeed,digideskio/ngx_pagespeed