Signal-analog

* Dashboard resources have learned how to check for themselves in the
SignalFx API, and will no longer create themselves if an exact match is found

* Adds support for base Resource object. Will be used for Chart/Dashboard
abstractions in future versions.
* Adds support for base Chart and TimeSeriesChart objects. Note that some
TimeSeriesChart builder options have not yet been implemented (and marked
clearly with NotImplementedErrors)

* Adds support for function combinators like `and`, `or`, and `not`

* Add README documentation

* Initial release

[deployment]: https://github.com/Nike-Inc/signal_analog/wiki/Developers-::-Deployment
[migration-1x]: ./docs/migrating_from_1.x.md