Provide a pimp-my-library class for `Wallet`

This pimp-my-lib class extends the functionality of bitcoinj's `Wallet`
in order to:

* Have some accessors that returns `BitcoinAmount` instances rather than
`BigInteger` objects representing Satoshis.
* Have a function `blockFunds()` that may be used to block funds. This
will commit the tx marking its funds as spent.
* Have a function `releaseFunds()` that may be used to release
previously blocked funds.