Added the Skytools extended transaction ID module to contrib as discussed

Added the Skytools extended transaction ID module to contrib as discussed
on CORE previously.

This module offers transaction ID's containing the original XID and the
transaction epoch as a bigint value to the user level. It also provides
a special txid_snapshot data type that contains an entire transactions
visibility snapshot information, which is useful to determine if a
particular txid was visible to a transaction or not.

The module has been tested by porting Slony-I from using its original
xxid data type.

Jan