Multiset

Performance improvements.
Multiset now avoids calls to `isinstance` with ABC types (they are costly) by checking the most common types (`list`, `tuple`, `dict`, `set`, etc.) directly first.

This version has some bug fixes (e.g. the ones discussed in 1).
Also, the test coverage has been improved as some features were untested previously.

This release splits the original Multiset class into two variants, one which is mutable (Multiset) and one which is immutable and hashable (FrozenMultiset). The Multiset does not inherit from dict directly anymore, so some dict methods are missing and some have been renamed. Mostly, the API remained compatible with version 1.0 however. This release is also compatible with Python 2.7.