fix tar bombs

[ghost]

this changes the release targets in the Makefile to use a subdirectory
of the format BINARYNAME_GITREV so extracting the archive results in a
single directory.

---

• I have signed the CLA

closes #160

[thebaer]

What do you think if we name the directory writefreely instead of including the version info? That should make installation / upgrading easier by maintaining existing paths, e.g.:

tar -C /var/www -xzf writefreely_0.10.0_linux_amd64.tar.gz

would always produce /var/www/writefreely.

[joyeusenoelle]

The only problem I can think of is a rare edge case where someone wants multiple versions installed, but they can do that manually so I'm not sure we need to care about it.

[ghost]

sure, sounds good to me

[thebaer]

That fixes it! Looks good -- thanks @robjloranger.