Preview Pull For Mining Guide

Created this to help keep track of work being done before the pull request against the main bitcoin.org repository.  Feedback both here and on the mailing list is welcome.

**Warning:** the commits shown on this pull will be squashed down to one commit per author for the real pull.
I agree this is a fundamental programming principle.  However, it seems to me that Javascript is already doing the right thing here by displaying "NaN" in the result box when a non-numeric is entered, providing feedback to the user that they've done something wrong.  This will probably work even better when I add the extra event handlers you suggested for immediate feedback.