Srcrefs

Hello Hadley,

As proposed, here's a pull request (proposal #1) that implements the srcref automatic fetching from expect_that. 
It is given to the reporter in result$srcref, that is given to the reporter. 
I also modified SummaryReporter to include the test file and line number for failures.

It should work on both R-2.15 and R-3.0, I avoided the call to parse(keep.source=TRUE) by using the option (and restoring it) instead. There should not be side-effects.

Best,
Karl