ENH: Added support for date attributes to io.arff.arffread

Adds support for date attributes in ARFF files. The date format according to Java's SimpleDataFormat is converted to Python's datetime.datetime.strptime format. Data is converted to an array type numpy.datatime64, where the datetime unit is determined from the format string. Currently, the finest time unit are seconds. SimpleDataFormat would support milliseconds, datetime.datetime.strptime only microseconds, though, i.e. there is no one-to-one correspondence.

This requires numpy >= 1.7