allow snappy_compression as a default option in test_options.py::TestOptions::test_simple

Allow to compile the extension everywhere

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

OSX support for building, simple update in setup.py

Update installation.rst

Missing steps need to be added for clean installation.

Merge options source_compaction_factor, max_grandparent_overlap_bytes and expanded_compaction_factor into max_compaction_bytes