Fix play_get_metadata_prop docstring

Function should be "playerctl_player_get_metadata_prop"
Bugfix: correctly emit "stop"

When playback status becomes "Stopped", the player signal "stop" will
correctly be emitted instead of "pause"
Add the player "exit" signal

The "exit" signal is emitted when the player exits.

We determine when the player has exitted when the PlaybackStatus
property has been invalidated.
Implement player memory management

Player now has dispose and finalize GObject class methods which will
free resources related to the DBus proxy and its own properties.
Bugfix: consistently get/set player_name

Formerly the player_name property getter would return the bus name, and
the property might not be set at all if the player was created without a
player_name.
Comment out the properties-changed signal

This signal is not implemented yet and is only used for debugging
purposes.

This signal might not be implemented in favor of targetted events for
convenient scripting.
Add link to releases on README
Add stop cli command to readme
Bump to version 0.3.0
Add chutzpa to readme joke