Prefixeddict

Added supported for the 'del' operator and a missing .clear() method.

This release has the following changes:
- Improved compatibility with standard Python dictionaries: the 'in' operator now works as you would expect, and iteration is done on the PrefixedDict's keys by default.
- Added full laziness in order to minimise memory usage. This works on Python 2 as well, as it uses itertools.izip on those releases.
- Fixed a bug that would cause different prefixes to prefix each other with themselves.

This is the first public release.

It does what it says it does on the README file, and is compatible with all Python interpreters it claims to be compatible with.