add ability to edit homepage and description of repo

This adds the following two commands:

* `!gh repo update description PagerDuty/lita-github The new lita-github description!`
* `!gh repo update homepage lita-github https://github.com/PagerDuty/lita-github`

As a note these two commands are enabled by default.There are two configuration options that can be used to disable this functionality:

```Ruby
config.handlers.github.repo_update_description_enabled = false
config.handlers.github.repo_update_homepage_enabled    = false
```
Merge pull request #19 from PagerDuty/gh_repo_description_and_homepage

add ability to edit homepage and description of repo