Please push release tags

[rillian]

The v0.3.1 release has a nice signed tag in the github repo, but no other releases do.

When auditing dependency updates, it's helpful if the upstream repo has clear tags for each published release, so simple git diff and git log can be used to review changes. It would be great if you could push tags for at least the recent releases to make this possible.

In the meantime, here are some corresponding commits according to the crates.io package metadata

version	commit id
0.1.15	aba3d3f
0.2.2	22e9297
0.3.1	c05c4e0
0.3.5	d0eedf2
0.5.0	98d7025
0.6.0	e5095a8
0.7.0	34293f5

Related to #91 and #104.

[sorpaas]

All tags have been pushed.