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.
Since this one is used only once, you could put it in the `get_multiplier` function. And just use:

    var hashes_per_sec={"GH/s" : 9, "TH/s" : 12, "PH/s" : 15};