Add seeking to rawread and macca, and tests

[pconerly]

@sampsyo

WIP towards #71 and #72 . On the path of adding seeking, I did a variety of things:

• I added block_samples into audio_open. It works in macca and rawread and I'm sure it's broken (via Unexpected keyword arg) in the other backends.
• Added unit tests for rawread and macca
• Added audio file fixtures for those tests
• Added a tox config to run in py27 and py36
• Added seeking to rawread and macca

Moving block_samples to the init for the other backends, and adding seeking to them is still needed. But let me know how this looks!

Todos:

• [ ] 1st round Code review fixes
• [ ] ffmpeg
• [ ] mad
• [ ] gstreamer