added support for blockcypher testnet

[SchulerSimon]

Description

Class added that supports the www.blockcypher.com test network. Blockcyphers test network has the version_byte set to 27

Type of Change

• [x] New feature
• [ ] Bug fix
• [ ] API reference/documentation update
• [ ] Other

Does this introduce a breaking change?

no

Are documentation updates required?

no

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}