Bug fixes

* 16-bit sample loader fixed. Fixes #14
* Envelope code is split from the sample code
* Volume envelopes that are switched off give a value of 0 when note is
decaying in sted of the given default value
* Volume of samples that ended is no longer set to 0. This fixes #9
* Volume envelope is taken into account when reporting player status.
Fixes #10
* Parameters for sample format and looping are placed in separate enums