Fall back to project version and write git info to version.clj

A couple of modifications:

* In case no git tag exists (which may be rather likely when working with snapshots), fall back to the version defined in `project.clj`
* Write more git info to `version.clj`:
```
    ;; Do not edit.  Generated by lein-git-version plugin.
    (ns insurrection.version)
    (def version "1.0-SNAPSHOT")
    (def info {
      :date "Wed May 28 14:18:10 2014 +0200"
      :author "Josh Glover <jmglov@gmail.com>"
      :revision "5f9a2307dd5692bf9fd18e60a1c9cd779dabaeab"
      :version "1.0-SNAPSHOT"
    })
```