Merge master to orm - Pony ORM release 0.5-beta

Pony ORM release 0.4.9:

1. Database create_tables()/drop_all_tables() methods
2. Database.drop_table(name), Entity.drop_table(), Set.drop_table() methods
3. Database.disconnect() methods (allows SQLite files deletion after disconnection)
4. Pony now automatically enables foreign key checks in SQLite
5. Entity.exists(...) method added
6. distinct() function added: select((s.group, sum(distinct(s.gpa))) for s in Student)
7. Refactoring & bugfixes

Conflicts:
	setup.py

Pony ORM release 0.4.8:

- use standard transaction mode by default instead of optimistic mode
- SELECT ... FOR UPDATE support added: select(...).for_update()[:]
- UUID datatype support added
- automatic foreign key indexes generation
- MySQL foreign key bug workaround added
- check_tables parameter of generate_mapping() is deprecated
- bug fixes

Pony ORM release 0.4.7:

- @db_session decorator is required for any database interaction;
- support of pickling/unpickling (queries and objects can be stored in memcached);
- lazy collections - don't load all the items if only one is needed;
- incremental query construction: q = q.filter(...);
- datetime precision now can be specified;
- multiple bugs were fixed.

Pony ORM 0.4.6 release - Psycopg2 support

Pony ORM 0.4.5 - Bugfix release

Pony ORM 0.4.4 - Bugfix release

Merge master to orm: update version to 0.4.3

Merge master to orm: Update version to 0.4.2

Update version to 0.4.1