Reporting load time to formatters

This is the start of an implementation of measuring load time and reporting it via formatters. I wanted to get feedback on roughly how this works before I went much further along with it. Basically the idea is to store the start time as soon as possible in config (which allows it to be overridden by spork or anything else), and then compare that to the start time on start.

At the moment this is a breaking change (because of the extra argument to start). I weighed this up against having an extra method, or adding in if's to work around it, but I think in the longer run it'd be better just to do this even if it means waiting until RSpec 3.

Thoughts? If this is generally agreeable I'll continue and start on the formatters.

This is in reference to #489