Added awkward.frompandas() for intuitive DataFrame -> Tables conversion

[NumesSanguis]

Initial support for a simple function that converts a Pandas DataFrame to an Awkward Tables. The new awkward.frompandas(dataframe) does:
return awkward.Table({name: dataframe[name].values for name in dataframe.columns})

Closes #215

[jpivarski]

This is a pretty simple function, and I approve! I don't require it, but you might want to add a simple unit test anywhere in the testing suite that makes sense. You can also increase the last digit of the version number to one higher than the latest release. That just means I'll be able to deploy it right away.

Since I have no objections, I can merge this anytime you're happy with it. But please do explicitly tell me when to merge, so that I don't merge it while you're making some last-minute change, like adding a test or something.

[NumesSanguis]

I also think a unit test is a good idea. I hope I'll get to it soon.