Improve multi-chain support for steem-python library

Bug Fixes

1. PR https://github.com/steemit/steem-python/pull/172 removes hardcoded "STM" prefix from internal key storage (wallet) and replaces it with dynamically-choosen prefix. So, internal wallet can now support another chains based on steem code but with different key prefixes.
2. PR https://github.com/steemit/steem-python/pull/173 fixes transaction signing with non-steem networks. In steem we're always have STM prefix for pubkeys, but in other networks prefix is different, so transactionbuilder just cannon find the proper privkey-pubkey pair whether prefixes does not match. This PR provides dynamically-choosen prefix to transactionbuilder.

New features

1. PRs https://github.com/steemit/steem-python/pull/171 and https://github.com/steemit/steem-python/pull/174 just adds GOLOS blockchain definitions, so library can work with it.