Fix serial communication on OS X. 

The current implementation of rs232, used to do IV curve, doesn't work on OS X. This appears to be an issue with different implementations of **termios**. Specifically, `tcsetattr` would return an error with the given port settings. 

The solution, which I have tested both on an iMac and my laptop running Ubuntu, is to leave the default port settings unchanged except for modifying the baud rate. This of course depends on the default settings of the port being acceptable, but this appears to be the case on the platforms I tested. 

This PR aims to resolve #168 .