Breaking API Change: Swap indexes of Wav.Data*. Add API for reading WAV in chunks. Added unit tests for 'wav' package

[nlefler]

Changes in this commit:

1. Changed Wav.Data8, Wav.Data16, and Wav.Data to be indexed by sample and then by channel.
2. Added API to stream a wav from an io.Reader rather than loading all data into memory.
3. Added unit tests for 'wav' package.

Change #1 is a breaking change to the current API.

[maddyblue]

Sorry I've taken so long to review this. I'd ask that you split these changes up into separate PRs, though, so they are more reviewable. Regarding each change:

1. What is the reason to change to sample-indexing instead of channel-indexing? Does it solve any problems or make more sense?
2. Fantastic. Move this to its own PR and it'll get merged.
3. I'm happy to see tests, but I'm not willing to use goconvey. It's a fine project that I've used before, but I prefer to stick to the stock tests API.

[nlefler]

Sure I can do that. I'll include reasoning for the index change in it's PR. Closing this.