added small fixed-key descriptor module

minor cleanup throughout codebase

add pure python hdwallet implementation (basic features) and removed …

…hdwallet lib dependency

new proxy implementation; removed previous dependency

add PSBT (BIP-174), tests and examples

updated ecdsa to >=0.19.1,<0.20.0

updated to v0.7.1

updated to 0.7.0 for block functionality

fixed deserialization of Script object for >75 bytes; issue #68

tag v0.6.6