Allow `rocksdb.DB` instances to be manually closed (second revision PR)

While `delete rocks_ptr` is a deterministic operation in C++, the Python analogue `del rocks_handle` is not – disposal and finalization of the Rocks database are entirely dependent on the Python garbage collector (q.v. the `python-rocksdb` tests themselves, which call `gc.collect()` after `del rocks_handle` to attempt to force the destructor to run). This change exposes a method to trigger the destruction of the underlying Rocks database pointer (deterministic!) through the Python Rocks handle; existing code will not need to be changed, as the Python object destructor (non-deterministic!) will now call this method.

This is the second revision of this PR – it resolves the first revision, #39.

Column Family Support

Add support for Column Families in a runtime safe way.
Add unittests to test functionality
Insure all unittests are passing.
Cleaned up unittests to not use a fixed directory in tmp, but use tempfile

Docs: Exporting CPLUS_INCLUDE_PATH, LD_LIBRARY_PATH and LIBRARY_PATH

Modified docs to export CPLUS_INCLUDE_PATH, LD_LIBRARY_PATH and LIBRARY_PATH correctly even if they weren't originally assigned

Remove `fetch=False` unsupported keyword from DB.iter{items,keys,values} documentation

Docs: fix typo in installation command line

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.