Add Injectable signing/encryption

[relativityboy]

As a web/mobile developer I want to be able to choose my own crypto libraries so that I can tune performance and overcome issues in default libraries should the need arise.

AC

• [ ] developer can inject their own, compatible, libs & dsteem will utilize them
• [ ] there are tests to cover this injection mechanism
• [ ] there is documentation showing how to inject a lib
• [ ] there is documentation defining the interface required of an injectable lib.

[relativityboy]

Closing because brilliant, but also slightly dumb.