Add automated support for GF > 2^8 + RSCodec.check() + unit tests

[lrq3000]

This PR adds:

• Automated support for galois fields higher than 2^8 (256).
• RSCodec has been refactored and a check() method was added.
• RSCodec supports multiple instances with different parameters and fields.
• More unit tests (coverage ~96% !).
• Unit test on Python 3.5.
• Faster Travis builds by using caching.
• Bump version 1.3.0 and updated setup.py from Beta to Stable. I think it's the correct time now, there are already regular users and no bug reported, so I think everything's ok.

Also, please note that this PR breaks compatibility for RSCodec.decode() (hence the major version bumping). It's because it now returns a couple decoded_message, decoded_mesecc where decoded_message is the repaired message just like before, and decoded_mesecc is the repaired message+ecc, which can be fed to RSCodec.check() directly to check that the decoding was successful.

[lrq3000]

No news Tomer? It's an important update, I just got a request about this feature (automatic support for galois fields above 256). Please test it and merge if it works alright for you! (it should, there are unit tests to ensure that!).