README: Correct link to Travis CI build status
cgmp: Add declaration of the necessary GMP functions in pxd file
cqsoptex: Add declarations of the necessary QSopt_ex functions in pxd file
qsoptex: Add pythonic module wrapping the C interface
Add configure checks for Python module and make rules

This change adds the `--enable-python-module` option to the `configure`
script, and the necessary checks for Python and Cython.