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.
Perhaps not as readable, but more compact, you could drop the get_multiplier function completely by using:

    var hashes_per_sec={"GH/s" : 9, "TH/s" : 12, "PH/s" : 15};
    ...
    multiplier = hashes_per_sec[percent_form.elements["percent_multiplier"]],