Collect timing data for each test

This adds a `resultTime` field to the `Result` type, which tasty will fill in after running each test. I updated the Provider functions to return a `PreResult` which is just a function of type `Double -> Result`. I also updated the `ConsoleReporter` to print the time each test took, as well as the time for the whole test-suite.

(I also snuck in a commit adding ghc-7.8.3 to the travis-ci build matrix)