Use built-in snappy library if system-wide snappy is absent

Now python-snappy can detect the absence of a system-wide snappy library and respond by building and linking to its own copy of the library. Also python-snappy will use its own copy if system library is older than 1.0.4. The python-snappy source distribution carries its own copy of the source code so that no network access is necessary for its install to complete successfully.

Limitations:

* windows not supported;
* pypy not supported;

Changes:

* include snappy source to python sdist and use it if system-wide version is absent;
* use tox and travis to run test suite for multiple interpreters, including py26, py27, py32, py33, pypy;