0.7.3: Merge pull request #1113 from talumbau/consump_dropq

Contributor	PR#	Description
Martin Holmer	#1081	add ability to use an expression to specify a policy parameter
Martin Holmer	#1083	expand scope of JSON reform file to include non-policy parameters
Martin Holmer	#1088	fix capital-gains-reform bug reported by Cody Kallen
Martin Holmer	#1098	add ability to conduct normative expected-utility analysis
Cody Kallen	#1104	add ability to compute MTR with respect to charitable cash contribs
Zach Risher	#1092	provide Pandas 0.19.1 compat by fixing dataframe.to_csv() usage
Zach Risher	#1094	unify environment definition by removing requirements.txt
Matt Jensen	#1109	reorganize current_law_policy.json and add section headers
Cody Kallen	#1084	add dollar limit on itemized deductions
T.J. Alumbaugh	#1111	add testing for Windows with Appveyor