Makefile: add a "release" target

The release target in the Makefile will take care of generating the
manual page, and also create an i3blocks-version.tar.gz gzip'ed tarball.

The tarball will extract files to i3blocks-version/ directory.

I included the .1.ronn file as well, if you want to exclude it from the tarball, you can add 

```
--exclude=$(PROGRAM).1.ronn
```

to the tar command