v0.2.0

Release notes:

- Three dependencies have now been moved into `github.com/smimesign` directly:
  - `github.com/smimesign/certstore`
  - `github.com/smimesign/fakeca`
  - `github.com/smimesign/ietf-cms`

There are no other changes. The minor version bump delineates the presence of the new packages inside `smimesign` itself.

Author: lgarron