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/connection.py

@@ -0,0 +1,48 @@
+from pymongo import Connection
+
+
+__all__ = ['ConnectionError', 'connect']
+
+
+_connection_settings = {
+    'host': 'localhost',
+    'port': 27017,
+    'pool_size': 1,
+}
+_connection = None
+_db = None
+
+
+class ConnectionError(Exception):
+    pass
+
+
+def _get_connection():
+    global _connection
+    if _connection is None:
+        _connection = Connection(**_connection_settings)
+    return _connection
+
+def _get_db():
+    global _db
+    if _db is None:
+        raise ConnectionError('Not connected to database')
+    return _db
+
+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.
+    """
+    global _db
+    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)
+