Travis testing

[yhoogstrate]

Dear @simon-anders,

This PR allows to integrate the repo with Travis-CI. You only need to enable testing for this repo with your account at the Travis-CI website. Every PR will then be automatically tested.

This PR:

• obtains all dependencies in travis via conda and conda-forge
  • swig, numpy, cython, flake8
• runs flake8 on files in ./tests/ and ./setup.py to test it for compatibility with the pep8 standard
• initiates a gitignore file that excludes certain build files from inclusion in git
• runs tests in the ./tests directory
  • currently only checking if compilation went fine and HTSeq's version can be obtained, but from here we can expand the tests
• is currently specific for python 2.7

We should not merge this until we have travis working under your account

[simon-anders]

I've merged it in and Travis gives a thumbs up: https://travis-ci.org/simon-anders/htseq/builds/197330701

Great! :-)