add the convenience class PolKitTracker

add the convenience class PolKitTracker

This class allows a mechanism to greatly reduce the syscall and IPC
overhead when checking whether a caller on the system bus message is
allowed to do a specific action. In a nutshell, this class caches
PolKitCaller objects and a) updates them when ConsoleKit emits
ActivityChanged signals; and b) evicts such objects from the cache
when the caller drops off the bus.

There's also an example, in examples/tracker-example/ that shows how
to use this. This example is referenced in the API docs too.