Release 0.6.8

[MattHJensen]

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

The last PR included in 0.6.7 was #945. The notes issue for tracking release 0.6.7 was #863.

Here's an example

| Contributor         | PR# | Description |
| :---------------- | :--- | :----------- |
| Sean Wang | #858 | add extra income tax brackets and rates |

Which will be rendered like this

Contributor	PR#	Description
Sean Wang	#858	add extra income tax brackets and rates

[martinholmer]

Significant contributions to Tax-Calculator release 0.6.8 include:

Contributor	PR#	Description
Matt Jensen	#867	add ability to simulate reforms that limit benefit of itemized deductions
Cody Kallen	#972	add investment income exclusion policy parameter
Martin Holmer	#973	add ability to eliminate differential tax treatment of LTCG+QDIV income

[martinholmer]

Should the information in issue #949 be incorporated in release 0.6.8? After doing that, it seems as if #949 should be closed.

[MattHJensen]

I'll do that. Thanks for the reminder @martinholmer.

[MattHJensen]

0.6.8 release description updated. Closing.