Added a nullHandler function to deal with blank line input.

[dekay]

This change recognizes when the user hits return on a blank line. I needed to handle this case for my project (a blank line is used to wake the device I am emulating, after which it sends an ack() response), but I can see it coming in useful in other applications.

[dekay]

Added a second commit that adds support for a second line terminator. In my application, I needed to handle lines that terminated in either 0x0a or 0x0d.

This probably should have been done in a separate branch from the other change. Sorry about that. I'm a git newb.