Allow for generating from a custom file path
Add khronos-api submodule
Remove pre-generated lib files
Regenerate the loader source whenever make is called

The following variables can be customised when compiling the library:

~~~make
GL_NAMESPACE        ?= gl
GL_API              ?= gl
GL_PROFILE          ?= core
GL_VERSION          ?= 4.3
GL_EXTENSIONS       ?=
GL_FULL             ?=
GL_XML              ?= $(DEPS_DIR)/khronos-api/$(GL_API).xml
~~~

For example:

~~~
GL_VERSION=2.1 GL_PROFILE=compatability make lib
~~~
Update documentation
Make the gl-rs libdir
Merge pull request #53 from cmr/gen

Fix up of #52
fixed for latest rust and included+fixed pull request #54
Merge pull request #55 from HeroesGrave/master

Fix for latest Rust, and include+fix #54
Update glfw-rs
Make submodule updating a separate target
Merge pull request #56 from bjz/submodule-update

Submodule update
Fix cargo lite
Merge pull request #57 from cmr/update-master

Fix cargo lite
update to latest rust and fixed examples
Merge pull request #58 from HeroesGrave/master

Update to latest Rust
.travis.yml: Download rustc from rust-lang.org