Make installation more robust

* Cython is no longer required to build the cglpk extension
* ez_setup only used when required (and ships correctly)
* extras_require updated
INSTALL.md gives beta instructions (temporarily)

The instructions listed are for v0.3.0, so they recommend the use of pip
install --pre (for now) so the beta will be installed. This should be
revereted when the final version is released, and these instructions
will work correctly without the --pre tag.
setup uses pep440 compliant version
Merge pull request #97 from aebrahim/install

Improve installation