Better __repr__ for LuxDataFrame

[dorisjlee]

__repr__ is currently empty to facilitate better display(..) of the luxWidget. We should support string __repr__ of LuxDataFrame for string operations on the LuxDataFrame.
One toy example of where this shows up is when dataframes are put into a list, the string __repr__ is triggered:

Ideally, we should have a string version that looks more like this:

[dorisjlee]

Closed with #308