2 Small Updates

Dear Raja,
Just starting to use your toolkit, and have made a couple of small changes:
1. minor change to rdsamp() to the record regexpressions to correctly handle
alphanumeric record names (like http://physionet.org/physiobank/database/aami-ec13/)
2. updated the plot_data() function to handle data with less or more than 2 signals

Hope you can use them, and if you like to work like this, I will continue to contribute whatever changes I make that I think are also useful for you.

Best,
Rob.