Added User-Agent for requests.

``` 
$ gh-auth add USERNAME 
```
seems to be failing with the following error:
```
/gems/github-auth-0.2.0/lib/github/auth/keys_client.rb:27:in `block in keys': undefined method `fetch' for #<String:0xb7eef0f4> (NoMethodError)
```

This is due to the github api requiring a User-Agent to be sent with all requests.
 
#keys is undefined because the reponse from github is:
```ruby
=> "{\"message\":\"Missing or invalid User Agent string. See http://developer.github.com/v3/#user-agent-required\"}"
```

This PR adds a User-Agent with the following format: 'github_auth-VERSION_NUMBER' to all requests it makes.

PS. Thanks for this gem!