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.
A good practice here would be to always validate the value and convert it to a number (never trust data from the user). Non-HTML5 browsers may allow letters since the "number" input types is a HTML5 feature. E.g.

    var rate = diff_form.diff_rate.value.replace(/[^0-9]/g,'');
    if(rate==='')return;
    rate=rate.toFixed(0);