Fix glaring issues regarding playlist track removal and add tests.

I had written 0 tests for removing tracks and had neglected to test this feature in actual usage. That is, until I realized there were massive problems with the behavior of the playlist when removing tracks. Everything has been fixed, and there are now five tests for track removal behavior.
Add a few more fixes a couple of bugs surrounding track removal.
Fix a slider bug.
Fix a bug where a removed track triggers a play event.
Add a way for Muxamp to recover current track state after going back and forth in the browser history.

Using track index alone to store playlist state is not a comprehensive solution, because it is a pointer, not a "primary key" in the sense of uniqueness.

For instance, imagine the following sequence of events:

1. Current playlist track index is 3 of 4.
2. Track 2 is removed. (I: 3 -> 2)
3. User hits back in browser. (I: 2)
4. User his forward in browser. (I: 2)

The current track should be back to 3, but it is instead 2. Because we have not tied the current playing track to its index, we have no way of knowing which track is the current track without checking each one.
Update package.json and bump version number.

The bugs fixed in this version--regarding track removal--are severe enough that a version change number is warranted.