Add README in m4 to keep directory in git

This is to work around a bug in autotools. The m4 directory needs
to be present before `autoreconf` is run in `bootstrap`.
configure: Add check for Exuberant Ctags
Add Travis CI build instructions file
Update README with build status image
Merge branch 'travis-ci'
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.