#82 Add operators to get elapsed time

[Martin Büttner]

Add extended operators akin to Matlab's tic and toc. tic remembers the current timestamp in some internal state and toc returns the time elapsed since the last tic. Ideally there would be an implicit tic at the beginning of the program.

One could make them even more powerful by giving the timers created by tic names, so that you can do nested time measurements, but I don't see how that would be possible without always requiring a name, which would make the basic use case longer again, so I'd rather not have that, or defer it to separate operators.