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 closures for monitor functions to ensure thread safety.
Add kind markers to Monitor type

This monitor handles from potentially being used in unsafe ways.
Ensure Monitor handles aren't stolen
Allow monitor getters to return from their closures
Merge pull request #129 from bjz/monitor-api

Use closures for monitor functions to ensure thread safety
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