Reuse connections to well-known buses

This mirrors the behaviour of libdbus. Consequently, ConnectSessionBus
and ConnectSystemBus are renamed to SessionBus and SystemBus,
respectively.
Rework API for asynchronous calls

Similar to net/rpc, the new method (*Object).Go calls a method
asynchronously, taking a channel on which a structure containing the
response will be sent. That way, a single channel can be used for
multiple calls.

Fixes #17.
Improve and correct documentation
Allow calling methods without giving an interface
prop: Correct introspection data