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/document.py

@@ -1,7 +1,5 @@
 from base import DocumentMetaclass, TopLevelDocumentMetaclass, BaseDocument
 
-#import pymongo
-
 
 __all__ = ['Document', 'EmbeddedDocument']
 
@@ -15,3 +13,5 @@ class Document(BaseDocument):
 
     __metaclass__ = TopLevelDocumentMetaclass
 
+    def save(self):
+        self.collection._save_document(self._to_mongo())