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

Requires numpy >= 1.7
Throw exception of date attribute contains time zone information

Updated unit tests
Skip datetime tests if numpy < 1.7.0
Skip setUp when numpy < 1.7.0
Merge pull request #3641 from sebp/arffread_date

ENH: io/arff: add support for date attributes to io.arff.arffread