Add TryCast db function

Author: OskarPersson

setup.py

@@ -17,7 +17,7 @@
 
 setup(
     name='django-mssql-backend',
-    version='2.2.0',
+    version='2.3.0',
     description='Django backend for Microsoft SQL Server',
     long_description=open('README.rst').read(),
     author='ES Solutions AB',

sql_server/pyodbc/functions.py

@@ -0,0 +1,5 @@
+from django.db.models.functions import Cast
+
+
+class TryCast(Cast):
+    function = 'TRY_CAST'