Release 0.6.7 changes

[MattHJensen]

Please update this issue with your significant contributions as they are merged to master.

The last PR included in 0.6.6 was #857. The notes issue for tracking release 0.6.6 was #841.

[martinholmer]

Here is my notion of "significant contributions" since the version 0.6.6:

Contributor	PR#	Description
Sean Wang	#858	add extra income tax brackets and rates
Martin Holmer	#874	correct Net Investment Income Tax (NIIT) calculation
Martin Holmer	#898	correct Schedule R credit calculation
Anderson Frailey	#904	add ability to simulate Fair Share Tax, or Buffet Rule, reforms
Sean Wang	#913	add ability to tax pass-through income at different rates
Martin Holmer	#914	remove logic for expired First-Time Homebuyer Credit
Matt Jensen	#926	add itemized-deduction surtax exemption policy parameter
Sean Wang	#939	add ability to simulate high-AGI surtax reforms

[MattHJensen]

Thanks @martinholmer. Here is the release: https://github.com/open-source-economics/Tax-Calculator/releases/tag/0.6.7.