Modified for Linux compatibility.

Added image, test/images/help.png for testing under Firefox.
Modified test/test.rb to handle different OS flavors, supporting
a firefox example.  The test is still far from platform neutral.
Modified lib/sikuli-server/client.rb. Requires the environment
variables SIKULI_SCRIPT_JAR and JAVA_HOME to be set prior to running.
Previously the client was hardwired for Mac OS use.