v0.7.1

[pzivich]

v0.7.1 update

Stochastic Treatment IPTW #97

• [x] Adapt IPTW code and syntax from stochastic g-formula

• [x] Update ReadTheDocs

• [x] Details in documentation

• [x] Write tests for TravisCI

IPTW API change #102

• [x] Add warning to IPTW regarding the upcoming change. Will make the change in v0.8.0

[pzivich]

Build failure is due to scipy==1.3.0rc since it looks like it has some trouble. Need to figure out how to get Travis-CI to ignore RC