Tweaks to README and CITATION

Added some badges and moved some things around in README.
Run apt-get update in Travis

As suggested in https://github.com/travis-ci/travis-ci/issues/2124
Auto set version string and date at build time

Uses versioneer to get the version number from git and make the
version.c file from a templace. Also sets the year for the copyright
notice.
Missing still support for the docs and dist version strings.
Merge branch 'version-generator' of github.com:leouieda/tesseroids into version-generator