Remove uninstall/install targets

Clients should use git submodules instead
Make output cleanups

Don't display link.rs during link target
`-C link-args` isn't needed for compilation of library and examples

An optional `GLFW_LIB_DIR` variable allows for customising the location of the glfw binary.

For example:

~~~
GLFW_LIB_DIR=path/to/glfw/lib/directory make
~~~
Remove unnecessary target dependency
Update README
Update .travis.yml

GLFW shouldn't need to be built as a shared library any more. Test this in the Travis build.

Also, to speed things up slightly, we don't build GLFW's libraries, examples, and docs.
Merge pull request #143 from bjz/make

Assorted Makefile and README imrovements