Tests and bugfixes.

I wrote 66b66334 and 2bd0976db, then realized that I had merged a pull
request. So when I rebased, I didn't run the tests, because I was at the
airport, and it turns out that things broke. This commit fixes that.

1. Map now respects #on_value. There was a bug where the callback would
   get the original event rather than the mapped event.
2. Tests for select respecting #on_value.
3. Fixing the implementation of both of those. :'(

There goes my Travis streak, I bet... yay for commits at 10,000 feet!
Refactor test codes.
Merge pull request #8 from larrylv/refactor-tests

Refactor test codes.
Fix bugs with Stream derivative #on_value

The Map and Select Streams were not respecting the change
from an @on_value callback to a @callbacks list (my bad).
I've moved this logic into a protected #occur method on Stream
to make extending Stream cleaner.
Add coveralls.

I like that it comments on pull requests...
Let's not coveralls our tests.