Added teamcity reporter

Please take a look at this change to add a TeamCity compatible reporter for testthat. This change allows teamcity to correctly report the tests and keep track of success/failure over time.
It does this by using the TeamCity messaging framework, which picks up specifically formatted messages in standard output. (See http://confluence.jetbrains.com/display/TCD7/Build+Script+Interaction+with+TeamCity for details)
I have tested this and I am currently using this reporter in a continuous delivery environment.

Neil.