fix CachingHTTPFileSource::clearLoop
Merge pull request #721 from mapbox/clear-loop

fix CachingHTTPFileSource::clearLoop
disable uv_thread_self() asserts
add cli tool for rendering a map to an image
show default values and allow setting the output filename
add xcode project for render cli util
allow setting the cache database
don't provide a default value for the stylesheet file
fix crashes when running in non-continuous mode
set rendering mode so that we can skip intermediate renders for static images