Add possibility of delete a tag

As request on issue #121, here is a PR to add the possibility of delete a tag.

In a nutshell:

* Tag deletion will remove Check / Tags relation first, in order to avoid magic tag re-spawn
* Tag deletion action is available in the tag show page, red button in the top-right corner
* Tag deletion is also available on API, please see the Readme.

Thanks for taking a look!