Added render context and moved some of the functionality from window into it.
Merge branch 'master' of https://github.com/bjz/glfw-rs into csherratt-render_task

Conflicts:
	src/lib/lib.rs
Clean up formatting
Improve wording of doc comment
Allow for the creation of multiple rendering contexts for one window
Clean up render_task example
Add some more library documentation
Group Drop implementations with their respective types
Remove unused lifetime bounds
Add missing imports to documentation examples
Use a shared channel to detect if all the render contexts have been dropped.
Renamed make_context_current to make_current
Changed println to error
Update example
Merge pull request #128 from bjz/render-task

Add render context for multi-threaded renders