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