Pandas 14

cc @spacecowboy, this PR sets pandas to be strictly >= 0.14 (as mentioned, I'm okay with forcing people to use the latest version of pandas, as lifelines is so heavily tied with pandas). 

Tests pass locally with these few changes. Can I get a quick look from you?

I'm also bumping the version: once a decision is made on the fortran parts, I'll upload to pypi.