add simple version flag

With this commit, `./avian -version` just prints `Avian: 1.0` and exits.

In the future, we should print version info about the classpath we're using.

I didn't do that here because I didn't find a really clean way of doing it. We could:

Add a java class that prints classpath info (e.g. avian/ClasspathVersion)
Use jni to invoke System.getProperty
Use lower-level avian-specific APIs to get either system properties or classpath version info directly.
I don't like the first two because they're fragile (particularly in the presence of proguard), and the last way is not portable.

@dicej, thoughts?