Added SVN logging observer.

It isn't triggered by anything yet. When the observer is called, it uses
Blinker to send out a signal for every [T12345] marker it sees in the
first line of each commit log. Those signals aren't connected to anything
yet.

NOTE: this requires the 'svn' Python module , which is a wrapper for the
'svn' commandline client. This client needs to be installed on our docker
when we deploy.

requirements.txt

@@ -1,9 +1,10 @@
 # Primary requirements:
 pillar
+attrs==16.1.0
+svn==0.3.42
 
 # Testing requirements:
 pytest==3.0.1
 responses==0.5.1
 pytest-cov==2.3.1
 mock==2.0.0
-