Add an api version number to allow libraries that implement ops for

theano/__init__.py

@@ -41,6 +41,14 @@ from theano.configdefaults import config
 # Version information.
 from theano.version import version as __version__
 
+# This is the api version for ops that generate C code.  External ops
+# might need manual changes if this number goes up.  An undefined
+# __api_version__ can be understood to mean api version 0.
+#
+# This number is not tied to the release version and should change
+# very rarely.
+__api_version__ = 1
+
 from theano.gof import (
     CLinker, OpWiseCLinker, DualLinker, Linker, LocalLinker, PerformLinker,
     Container,