Rename AmountUnit constants.

Besides being shorter, using "BTC" rather than "Bitcoin" in the
AmountUnit constants is deemed to be better for these units as BTC is
already a recognized monetary unit.

AmountBaseBitcoin has likewise been renamed to AmountSatoshi as this
is consistant with how it is returned as a string.  The "standard"
part of the comment in the const block has been removed, as Satoshi is
technically not a standard term for this unit.

ok @davecgh