add mock functions

for `multisig_pubkeyhashes_to_pubkeys` and `decoderawtransaction`.
With these two mock functions we don't need any more to have a bitcoind instance running.
Note that like `getrawtransaction` mock, `decoderawtransaction` use `test/fixtures/rawtransactions.db` database. To populate this database we need to have a bitcoind instance running and execute one time tests with `--initrawtransactions` and/or `--saverawtransactions`. 
For instance `py.test --skiptestbook=all --initrawtransactions --saverawtransactions`.