Fix NumberFormatException if port is saved as blank string

if you open the port option and click ok, it saves a blank string in
preferences and this gets loaded instead of using defaultPort so throws
the exception trying to convert "" to an int.
Merge pull request #14 from tombriden/blank_port_fix

Fix NumberFormatException if port is saved as blank string
Merge branch 'master' of https://github.com/erickok/transdroid into tom_build