Some tests on arff format and small correction.

Hi,

I added and ran some tests (test_loads.py) on arff format according to the ARFF document (http://www.cs.waikato.ac.nz/ml/weka/arff.html) and put a patch (arff.py) for a remaining problem which I found with attribute name starting with non-alphabetic character. 

Actually, I had found even more problems last year with arff format last year and sent a pull request with some testcases but accidentally had dropped it at the time. Returning from an absence, I found that your project have been improved not only in a way of code structure but also by having test cases, and came up with the format problems that I met last year. Glad to see that quotation problem on attribute / relation name has been resolved but a trivial problem still left for me to fix. :-)