Performance improvements for M2M properties

As described in PR #44, this release's focus is mainly meant to keep the number of DB queries constant when cloning objects including M2M fields.
Previously, the complexity was O(n) with n being the number of M2M relations. Thanks to @raphaelm complexity could be fixed to a constant value (so, O(1)).

Lowercase DB column names and more Django-like intermediary tables

This release covers PRs #39 and #40.
It enforces lowercase DB table column names and a correct DB namespace.

!!!DBs created with prior versions (<1.4.0) of CleanerVersion will not be compatible with this and future versions (>= 1.4.0)!!!

M2M assignment of absolute values

Assignment of absolute lists to M2M related properties is now possible. E.g.:
up to now, only assignments like "author.books.add(book1)" were possible.
With this feature, assignments like "author.books = [book1, book2, book3]" are possible.

v1.2.3: Merge pull request #14 from swisscom/reduce_warnings_w_django17

Reduce warnings w django17

v1.2.2: Merge pull request #10 from swisscom/bugfix/null_in_m2m_rel

Bugfix: None in M2M relationships

v1.2.1

Added support for Null-ForeignKey relationship instances

v1.2.0: Merge pull request #7 from swisscom/travis-integration

• Added Python3.4 support
• Code gets now checked by travis-ci.org

v1.1.2

Fixed setup.py script in order to add versions.utils as well; Exclude…

v1.1.1

Set version to 1.1.1

v1.0

Precisions on how to embed and use CleanerVersion