Add support for date attributes

[NateLanza]

Display of date attributes from Multinet is not currently supported and needs to be. Here's a summary of what should happen:

• The attached branch, date-atts, removes the processing filter that currently keeps atts with the date type out of Upset. Removing this filter only adds a blank column for date atts
• You'll need to figure out the range of possible formats for date atts coming from Multinet. This is probably very expansive, as a user can define any column as date when importing data, so you'll probably have to decide on some restrictions and figure out how to handle exceptions in the processing step.
• Date strings should be transformed to Date objects (JS Native) during processing
• You'll have to figure out a good way to display dates in the attribute scale for date columns. This is easy for dates that are just years (like ReleaseDate from the Movies dataset), but much harder if you need days or times.
• Once you have the scale figured out, you can use the same attribute plots to display date distributions, but you'll have to figure out how to get the dates lined up correctly with the scale; I anticipate this will be harder than for numerical atts. You'll want to re-use the components for attribute plots; dates should probably be transformed into numbers before being passed to the plots.