Add ifdefs for compatibility with Player < 3.1

The Stage plugin contains some features (graphics2d multiline,
etc.) that were added in Player 3.1.  This commit adds some
code to let the latest version of Stage compile against Player
3.0.2 and earlier.
Add camera interface support to the Player plugin

This commit adds camera interface support to the Player plugin,
based on code originally provided to the project by Kevin Barry.
Images from Stage camera models can now be accessed by Player
clients like "playercam"
Merge pull request #32 from jpgr87/camera

Camera interface for Player plugin