Tests: add new unit tests for SlaveStatus

A few more tests in order to test Buildslave.status ... yes it's going away, but doenst hurt to have tests :)
Slave: add 'info' dictionary

Make the 'info' dictionary more public and provide hooks to update it and set it
SetSlaveInfo step to correspond to SetProperties
Interpolate("%(slave:XXXX)s")
SlaveInfo: doc in progress
Fix Travis issue: duplicate function.

The other function seemed bogus and I think was a relic.
SlaveInfo: fix pep8 issues
Merge branch 'SlaveInfo' of git://github.com/jaredgrubb/buildbot

+autopep8