Abstract out the notion of clocks

Please consider the attached pull request that redefines "now" and "today" in terms of a new "system-clock" class. The purpose of this re-factoring is to make it easier for other libraries (e.g. time mocking libraries) to play around with the current-time. For an example, see [Delorean](https://github.com/cddr/delorean) which provides "with-frozen-clock", "with-shifted-clock", and "with-scaled-clock" macros.

The other changes are:-

a) The addition of instructions for loading the library via quicklisp
b) Simple instructions on how to run the Stefil tests

If you like the idea, I'll update the pull request with a documentation patch as soon as I can.