 pd.get_dummies(s) with s being a logical test
Hi, 

I am using Pandas more and more for preparing my dataset before feeding it to other statistical software. 

I believe something really useful and, somehow, missing from the current version of Pandas is a simple way to generate dummy variables based on a logical test. 

Dummy variable coding (0-1) is everywhere in the academic literature (I have a phd in economics) and are a key component of many statistical tests.

Something like 

`pd.get_dummies(df.A > df.C)` that would generate a dummy that is one if A>C and zero otherwise, allowing the user to 
- include/exclude missing variables
- specify what happens when columns have mixed types (see (https://stackoverflow.com/questions/36373681/how-to-create-dummy-variables-in-pandas-when-columns-can-have-mixed-types)

Of course, we can always generate the dummies above manually, but the experience shows that this simply dummy variable creation can be, at times, tricky.

What do you think?
