Adds dlen and doff keywrods arguments to DB.get and DB.put, allowing partial gets and puts:
1. Defines a new C function add_partial_dbt that adds DB_DBT_PARTIAL flags and sets dlen and doff in a DBT struct, based on PyObjects dlenobj and doffobj passed to it from the appropiate functions (DB_get, DB_put).
2. Added unit tests to test_basics.py.
Still missing is cursor support, and support for large values of dlen and doff. A higher level file-like interface would be useful as well.