Add github issues support

This adds GitHub issues as a supported service to fire off notifications too. Here is an example:

```ruby
require "comsat"

client = Comsat::Client.new
client.create_route(
  "notify_on_test",
  ["github://APIKEY:X@github.com/asenchi/testrepo4"]
)

client.notify("notify_on_test", {
  :message => "Testing new GitHub Service",
  :source => "test-script",
  :message_id => "test-#{rand(1_000)}",
  :message_type => "notice"
})
```

Here is a [test issue](https://github.com/asenchi/testrepo4/issues/1) showing a "notice" and "resolve" notification.