Issue #11: parse values.

Also affects the definition of the "bitcoin" asset to default to full bitcoins.
Also affects the "balance" and "send" commands (not tested!)

Be gentle, I'm new and I'm not sure how you wanted to deal with rounding (there is rounding in both directions) or if you wanted unit to ever be less than 1 (or how unit would be inverted:  should bitcoin use 100000000 or .00000001).  Please let me know what you don't like about the change.  Also be careful, this drastically changes the behavior of the "bitcoin" asset.