Print total time for a build invocation

[alexcrichton]

I've found it nifty when building Servo that the total time for the build is printed at the end, and I think we may want to do so as well! Let's modify rustbuild (src/bootstrap) to print how long the entire program takes.

Depending on how easy it is we may want to modify the python script (src/bootstrap/bootstrap.py) as that's the wrapper for everything, but if it's too much effort then let's just modify src/bootstrap/main.rs and assume the python bits are fast.

Should be a pretty easy bug to knock out and I'd be willing to help out anyone interested!

[nikhilshagri]

I'll try my hand at this one.

[alexcrichton]

All yours @cynicaldevil!