Improve multiplication, add shortcuts, add pattern property, tidy up.

Done:

- Slightly tidy up code.
- Add `pattern` property for regex-s.
- Optimized & improved multiplication: now we do not duplicate code, but set number of repetitions. Exact, min and min-max numbers of repetitions are supported.
- Add `StringStart` shortcut.
- Add `StringEnd` shortcut.
- Add shortcuts word boundaries.
- Update readme.
- Update setup.py.

Things not done:
- `gh-pages` branch was not modified. It's not a big deal, so you can update it after merge.

Things to think about:
- It can be useful to add `Alphanum` and `Printable` shortcuts.
- Not sure if it's possible to implement a single shortcut for caseless literals, but this is an interesting idea too.
- It can be nice to add actions for groups, e.g. `.as_group('dollars', int)` or `.as_group('date', to_date)`. This can be tricky a bit and implementation will need to wrap default `MatchObject` to apply post-match callbacks.
- You may add integration with some services, such as Travis, Coverage, Scrutinizer, Codacy, Landscape, etc and add badges with current version on PyPI, so users will be more informed about state of the library ([example](https://github.com/IL2HorusTeam/il2fb-mission-parser/blob/master/README.rst)).