Scrap the multiple address book design.

The multiple address book design has been removed completely now. It was
simply complicating things.

Added a new method load() to ContactHandler() which takes care of
loading the ContactHandler() instance from a save file.

Fixed a small tiny in Shell().do_del().
Fix small bug in contacs.ContactHolder().format_phone().

The format_phone() method used to ommit that last pair on numbers by
slicing it out of the numbers list. Fixed that.
Add seperate exceptions for contacts and groups.

Added 4 new exceptions from ExistingEntryException and
UnknownEntryException, replacing Entry with Contact and Group. Also
removed the old *Entry* exceptions and made changes to contacts.py and
shell.py to use these new exceptions instead.
Fix formatting, punctuation and typos for docstrings inside contacts.py.
Version 0.2.6
ContactHolder().modify() takes a field and a value.

Contacts can now have their specific fields modifed, they
ContactHolder().modify() now takes a field and new_value argumnets and
so you can modify the field with this new value.

ContactHolder() fields are now stored under a dictionary to make
handling them easier and more convenient.

Modifed Shell().do_modify_contact() and
ContactHandler().modify_contact() to adapt to the new changes.
Revert "Version 0.2.6"

This reverts commit ce1d41f9ab39a8444f1a2a69868e5e351c1e8833.
Revision number