Replace all usages of `BtcAmount` by `BitcoinAmount`

All the places where `BtcAmount` type was used is refactored to
use `BitcoinAmount` instead.
Remove `BtcAmount class

`BtcAmount` is replaced by `BitcoinAmount` and no longer needed. We
can remove it definitely.
Remove all classes from `common.currency` package

All classes from `common.currency` package are no longer needed, and
therefore removed.
Remove `OldAmountMappings` and `NewAmountMappings`

The inner objects `OldAmountMappings` and `NewAmountMappings` are
removed. `OldAmountMappings` is no longer needed, and the mapping
defined in `NewAmountMappings` is moved to the scope of
`DefaultProtoMappings`.
Merge pull request #162 from apoloval/issue/141

Remove old `FiatAmount` and `BtcAmount` definitely