Add gzip of persistence, NewStashReader

This is a breaking change from v0.1.0 regarding the Unstash function.

Persistence now has gzip support of gob-encoded data before encryption. There is also NewStashReader providing a means to access an io.Reader of a Stashed store (post-gzip, post-encryption, without reading a persistence file).

Full Changelog: v0.1.0...v0.2.0