update version to 0.1.9

The major changes in this version are:
* A new TransactionBuilder class to make it easy to make any kind of
 transaction.
* BIP32 - hierarchical deterministic wallets.
* Further progress towards full test coverage and 100% compatibility with
 bitcoin core.
Merge pull request #207 from ryanxcharles/feature/version-0.1.9

update version to 0.1.9