Bip32 sign and verify now compatible with ElectrumSV and bitcoind

[AustEcon]

• Added bitcoinx-based sign and verify functionality to Bip32 class --> now signing and verifying messages on e.g. ElectrumSV or bitcoind match (which gives confidence to users of this library in my opinion).
• Added tests for Bip32 module (halfway done)
• Improved the behavior of the Bip32 module - there were a few odd quirks discovered when making the testing module.
• Removed travis CI support for python3.5 and pypy because pip kept failing with the new bitcoinx dependency... If anyone knows how to get travis to install bitcoinx - would be greatly appreciated!