Re-export GlCommandBuffer

Like GlDevice, GlCommandBuffer is needed by application code whenever
gfx::Graphics is used in a context where the type cannot be inferred.
Merge pull request #336 from hannobraun/master

Re-export GlCommandBuffer
Provide a common trait for getting slices from both meshes and buffers
Merge pull request #337 from bjz/common-slice

Provide a common trait for getting slices from both meshes and buffers
Move tests into root directory
Move examples into root directory
Remove tasks stub directory
Fix Makefile
Remove unused import
Merge pull request #340 from bjz/directory

Use a more ideomatic cargo directory structure
Move generated gl bindings into a separate crate
Move gl_device into separate directory

This will be converted into a crate once associated items are added and used in the `Device` trait. See rust-lang/rfcs#195
Merge pull request #341 from bjz/gl_device

Move OpenGL device implementation into separate directory
Fix crate name ambiguity
Merge pull request #342 from bjz/fix_warning

Fix crate name ambiguity
rename gl to gfx_gl
Merge pull request #343 from csherratt/master

rename gl to gfx_gl
Fix gl-init crate name
Move contents of renderer::front into root module

This makes the module structure more consistent with the device module.
Add screenshots of examples to their README