Add elapsedtime module

[jihunchoi]

Using progressbar module, I thought that it would be nice if clint has a module which measures time, in case of running a long single job.
Of course, that could be done using progressbar with a dummy length-one iterable object.
However, this module provides following features:

1. A timer run on explicit thread; this can show elapsed time even if the job is not finished.
2. Elapsed time can be simply measured using a simple 'with' statement.

I'm not sure if it is suitable to creating a new module for this feature, but I think this is useful for some users. :)