Added CollectionManager, made connection module

All connection-related functions are now in connection.py.

Created a ConnectionManager class for interacting with a collection
in the database. Top-level document classes have an instance of
a ConnectionManager (Document.collection).

Defined a 'save' method on top-level document's that uses the collection
manager's '_save_document' method to save the document to the database.

Added tests for CollectionManagers -- all unit tests now require a valid
connection to the database, which is set up in the tests' setUp method.

mongomap/__init__.py

@@ -2,39 +2,11 @@ import document
 from document import *
 import fields
 from fields import *
+import connection
+from connection import *
 
-from pymongo import Connection
-
-__all__ = document.__all__ + fields.__all__ + ['connect']
+__all__ = document.__all__ + fields.__all__ + connection.__all__
 
 __author__ = 'Harry Marr'
 __version__ = '0.1'
 
-_connection_settings = {
-    'host': 'localhost',
-    'port': 27017,
-    'pool_size': 1,
-}
-_connection = None
-_db = None
-
-def _get_connection():
-    if _connection is None:
-        _connection = Connection(**_connection_settings)
-    return _connection
-
-def connect(db=None, username=None, password=None, **kwargs):
-    """Connect to the database specified by the 'db' argument. Connection 
-    settings may be provided here as well if the database is not running on
-    the default port on localhost. If authentication is needed, provide
-    username and password arguments as well.
-    """
-    if db is None:
-        raise TypeError('"db" argument must be provided to connect()')
-
-    _connection_settings.update(kwargs)
-    connection = _get_connection()
-    # Get DB from connection and auth if necessary
-    _db = connection[db]
-    if username is not None and password is not None:
-        _db.authenticate(username, password)