Build fixes for Android windowing code

This begins porting the Android event loop to work with the inverted flow
control from #3761.  Unfortunately, GLUT does not give us enough control over
the event loop to really make this work, so this will build but it may not run
properly.  Our current plan is to get rid of GLUT and switch to Glutin in the
near future.

r? @pcwalton
Document bindings/utils.rs.
Document bindings/trace.rs.
Document bindings/callback.rs.
Implement the whole Attr interface
auto merge of #3935 : evilpie/servo/master, r=Ms2ger
Document bindings/conversions.rs.
auto merge of #3937 : Ms2ger/servo/docs-bindings, r=jdm
auto merge of #3925 : mbrubeck/servo/glut-loop, r=pcwalton

This begins porting the Android event loop to work with the inverted flow control from #3761.  Unfortunately, GLUT does not give us enough control over the event loop to really make this work, so this will build but it may not run properly.  Our current plan is to get rid of GLUT and switch to Glutin in the near future.

r? @pcwalton