Updated version number to match app version number (3.9.0).
Altered IOLocked and IOLockedMutex to use ReentrantReadWriteLock eliminating all synchronize() statements in the port IO methods.  This also eliminates 2 significant timing holes: where a read or write starts before the port is closed and the errant use of the non-atomic boolean closeLock variable.  ReadLocks allow for multiple concurrent ReadLocks but do not allow locking with a WriteLock.  A WriteLock cannot occur if ReadLocks are outstanding.  This is exactly what the authors pre-Java 1.5 were trying to accomplish albeit unsuccessfully.
Merge pull request #5 from PeteSL/master

Updated version number to match app version number (3.9.0). Altered RXTXPort to use ReentrantReadWriteLock instead of synchronize().