upgrade to newest dimple version dimple v2.1.0

This pull is primarily designed to upgrade to the newest dimple release [v2.1.0](https://github.com/PMSI-AlignAlytics/dimple/releases/tag/2.1.0) which provides new functionality documented in the release docs.

### Other Changes
1. fix bug in measureAxis sorting ramnathv/rCharts#506 in https://github.com/timelyportfolio/rCharts/commit/8d7e11284a0dec054bd58cbb3dbc2dce4ac9089f
2. allow use of interpolation options added in dimple v2.0 in https://github.com/timelyportfolio/rCharts/commit/8d7e11284a0dec054bd58cbb3dbc2dce4ac9089f
3. squash the tooltip css conflict with dimple and bootstrap #510 and #365 and dimple PMSI-AlignAlytics/dimple/#52
4. add `null` temporarily where dimple now expects `pieAxis` in `new dimple.series` in https://github.com/timelyportfolio/rCharts/commit/af6cb6397cb68a50c3a94dece0b21d62ed4f12f8 .  Address this in the next dimple rCharts pull.

All examples in examples.R passed.