Make all extensions sub-extensions of the gruel extension

[jwir3]

Right now, we have a number of extensions that the gruel plugin defines, including:

hipchat {
  ...
}

jira {
  ...
}

The problem with this is that it's ripe for conflicts if another plugin also defines a hipchat or jira extension.

We should make these sub-extensions of the GruelExtension, so users can specify the following instead:

gruel {
  jira {
    ...
  }

  hipchat {
    ...
  }
}