Major refactor simplifying and abstracting most components

* Refactored widgets in to a single widget that uses data attributes
  on the dashboard elements to determine which metric to get from the
  New Relic API and the limits for changing colour of the widget.
* Move logic for dealing with the API etc in to a new class, greatly
  simplifying the jobs.
Update README and copyright
Add list of metrics available from New Relic API
clarify custom dashboard HTML for metrics with spaces in their name
Add syntax highlighting for code blocks in README
Add screenshot
Merge pull request #9 from warrenguy/major-refactor

Major refactor