remodel v0.4.0

Callbacks are here! Now you can easily hook into various actions of your objects' lifespan and execute custom logic. For instance, mark a document object as draft before saving it; or set a new flag on a user object after initializing it.

Also, you can now get an object's key or update it with several fields at once, both in the spirit of remodel's dict interface of objects!

New features:

• introduced callbacks (#19);
• added get and update for objects (#3, #14);
• drop_tables as an extra helper (#16).

Improvements:

• moved helpers such as create_tables to dedicated module (#16);
• squashed various bugs (#10, #13, #17).