Frozendict

- Some little speed improvements
- Updated benchmarks

Some internal code cleaning.

- Speedup to creation of a copy of a frozendict
- Added a testcase
- Added benchmarks

Completely reworked. Now `frozendict` inherits directly from `dict` and does not use the slow `MappingProxyType` anymore. Immutability is preserved anyway.

Notable speed-ups in all methods.

- No more accepts unhashable values, like `frozenset`
- Added __slots__ for real immutability
- Better `pickle` support
- Some speed optimizations

First release