Allow harvesting non-integer quantities.

Some browsers (grrr Firefox for Android grrrr) assume "number" means
"integer" unless you specify `step="any"`. Perhaps the developers were
inspired by Kronecker?
http://mathoverflow.net/questions/34445/meaning-of-kroneckers-comment-to-lindemann

This was reported as a bug for non-integer weights; I've allowed
non-integer quantities in the other field, because some allowable units
(eg litres) are obviously divisible.
Explain step="any" in harvest _form view.
Test for spec=any in harvests/new.
Merge pull request #342 from pozorvlak/harvest_grams

Allow harvesting non-integer quantities.