Little tweaks to the JavaDoc of some methods.

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>
Fixed an error in the 'isBroadcast()' method of the 'TX64Packet' class,
when comparing the 64-bit destination with the 16-bit broadcast address.

Added the corresponding unit tests for the 'isBroadcast()' address
method for this class and the rest of packages.

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>
Updated the readme file to add the lisence for RXTX and SLF4J libraries.

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>