dbapi2 patching: do respect with statement on cursor objects (aws#17)

Some database backends (including already-supported `django` and not-yet-supported `psycopg2`) implement non-standard ability to use cursor in `with` statement like this:

    conn = get_db_connection()
	with conn.cursor() as cur:
		cur.execute('MY QUERY')

This syntax will automatically close cursor when leaving `with` statement.
The problem is that `Cursor.__enter__` returns `self`, thus dropping our `XRayTracedCursor` wrapper.

This PR aims to fix that issue by wrapping `__enter__` as well. If connection's `__enter__` returned itself then we return wrapper instead, else we return what cursor returned for compatibility.

Probably something similar should also be implemented for `Connection.__enter__` because some backends support it as well (including `sqlite3`).

aws_xray_sdk/ext/dbapi2.py

@@ -33,6 +33,13 @@ def __init__(self, cursor, meta={}):
             db_type = cursor.__class__.__module__.split('.')[0]
             self._xray_meta['database_type'] = db_type
 
+    def __enter__(self):
+
+        value = self.__wrapped__.__enter__()
+        if value is not self.__wrapped__:
+            return value
+        return self
+
     @xray_recorder.capture()
     def execute(self, query, *args, **kwargs):