Added getContext and getId statics

TransactionParticipants gets called by the TransactionManager using
their prepare, prepareForAbort, commit and abort callbacks, but for
situations where one needs access to the Context in a deeper class,
called by the participant (i.e. Managers), we now have a couple of
static ThreadLocal based methods:

 * Serilizable getSerializable ()
 * Context getContext (in case your Serializable is actually an instance of
   org.jpos.transaction.Context)
 * Long getId ()

Please note returned values may be null when run outside the TM life-cycle.

Please also note that the TM takes care of PAUSED transactions, setting
these values on the resumed thread.