Implement a COUNT ... BY aggregation operator.

This also removes the now obsolete scalar count() function and corrects the
expressions test naming (broken in
https://github.com/prometheus/prometheus/commit/2202cd71c9eb0477e218a7b854db1f35fa5427b9#L6R59)
so that the expression tests will actually run.
Merge pull request #233 from prometheus/feature/count-by

Implement a COUNT ... BY aggregation operator.