Extract `hubot-notifier-base` into utility hubot package

[patcon]

Moved from https://github.com/gittip/hubot-freshdesk-ticket-notifier/issues/2

Currently, there are a few things we're duplicating in Gittip's notifier packages:

• various levels of setting notifier room:
  1. allowing the setting of one global room envvar for all notifiers (not yet possible to implement)
  2. setting room(s) via envvar per notifier (override)
  3. notification-level overriding via querystring param
• allowing default and customized route (not sure the value in this... but saw it done in gittip's pagerduty notifier)
• allowing custom mustache templating via envvar

[patcon]

@parkr had a good idea to make the prefixes optional: https://github.com/gittip/hubot-github-repo-event-notifier/pull/17#issuecomment-43100568