Partial support for date and datetime formats

ARFF supports dates and timestamps. liac-arff currently doesn't support it, nor does scipy or the unmaintained arff package.

java.text.SimpleDateFormat looks awful to fully support, so I intend to implement partial support for now. In my application, I need to understand the following formats:

• yyyy-MM-dd'T'HH:mm:ss (the file format default)
• yyyy-MM-dd' 'HH:mm:ss (R's foreign::write.arff for timestamps)
• Any of the above with 'Z' appended (for importing/exporting localized times; a zero offset suits my needs)
• yyyy-MM-dd (R's foreign::write.arff for dates)