Replace version and commit-hash files with meta.json

Change from writing separate build and commit files to a single JSON file that can be a future home for other meta data about the project.

The new `meta.json` will live in the project's webroot, just like the current `version` + `commit-hash` implementation.

Here's what the JSON blob looks like:

```bash
$ cat src/NSAA.Umbraco/meta.json | json
```
```json
{
  "build": {
    "date": "2013-04-22 16:47:42 -0600"
  },
  "commit": {
    "id": "81276a8f84a58b7fbe5e126e3a520e1b0d68ff5f",
    "date": "2013-01-04T14:12:39-07:00",
    "author": {
      "name": "Erik Smith",
      "email": "erik.smith@imulus.com"
    },
    "message": "Merge branch 'develop'"
  }
}
```

Also adds `meta.json` to the .gitignore and removes `version` and `commit-hash`.