Preggy

New
- Added `preggy.__main__` (9)
- You can now use `python -m preggy` to print a quick reference of preggy's assertions

Documentation
- Added package docstring (hooray!)
- The `not_*` form of assertions generated by `create_assertions` no longer adds a (useless) docstring
- Fixed typo for docstring of `not_to_be_like()`
- README: Extracted "DEVELOPING.md" to its own file

Other minor changes
- Added `preggy.__meta__`; it contains all non-install related metadata

General
- The decorators `assertion` and `create_assertions` now use `functools` (5)
- `create_assertions` now reuses code from `assertion`
- Factored string formatting to `utils.py` (10)
- Added (minimal, debug) logging (8)
- This also involved refactoring `_registered_assertions` to use a slightly-modified `dict` class, found in `utils.py`
- `preggy.__init__.py` (correctly) no longer imports `fix_string`

Other minor changes
- Fix minor docstring typo in `assertions/comparison.py`
- Added 2 missing docstrings in `assertions.types.boolean`
- Many small edits to `README.md` for formatting and clarity