Allow to compile the extension everywhere

[sileht]

This change:

• uses to setuptools Cython automatic extension build system.
• Add tox.ini to run tests and build docs into virtualenv
• Add .travis.yaml and Dockerfile to run tests in CI
• Change requirements to ensure:
  • Cython and setuptools are installed before we build the Cython extension
  • tests dependencies are not installed by default
  • doc dependencies are explicit
• Add missing lz4 library
• Allow to build the module with any librocksdb headers (no-rtti)

Closes #15

[sileht]

Pull-request updated, HEAD is now e8de1ea7462d0650e923d9322e682a6f76704518

[sileht]

You can see the result of the travis CI: https://travis-ci.org/sileht/python-rocksdb/builds/339353563

[twmht]

@sileht

thanks! I will merge this PR in a few days.

[sileht]

Don't forget to enable travis on your repo to enable it :)