Subrequest streaming API

Streaming subrequest API - Added the ability to perform a 'streaming' subrequest, using ngx.location.capture_stream, meaning that the main request and the subrequest would run simultaneously. This is still preliminary, thus the feature is protected by a C macro. To enable it on compile time, the option '-DNGX_LUA_CAPTURE_DOWN_STREAMING' should be given to GCC.

For more information about the streaming subrequest API, go to
https://groups.google.com/forum/?hl=en#!topic/openresty-en/2M6Z9BNtufQ

The is a part of pull request #286