VersionUtil: Improve Android memory usage

The Android runtime implements Class.getResourceAsStream() with a very
memory-intensive cache, which holds the entire contents of the class's
.jar in memory.

A large Android app that uses Jackson will consume double the memory
today. Since memory is very tight on most Android devices, this diff
adds an alternative method to get version info for a package.

In specific, we now generate a class
com.fasterxml.jackson.core.json.PackageVersion at compile time.  At
run time, VersionUtil now checks the class's package to see if there's
a class named 'PackageVersion', and if so, reads its 'VERSION' static
field for the Version data for the package.  (If not, we still use
the existing VERSION.txt resource code as a fallback.)

I have separate patches for jackson-databind and
jackson-datatype-guava; it's easy to apply to other packages as well.

This is a nice performance improvement as well; we no longer incur all
the overhead of causing the runtime to open and cache the .jar
(unzipping it, etc.) just to read the VERSION.txt resource.

If the PackageVersion class doesn't exist, we'll of course fall back to the
existing test to get version info.

I confirmed using the Eclipse Memory Analyzer[1] that memory usage for
a large app I work on was reduced by about 5 MB with this fix (as well
as the jackson-databind and jackson-datatype-guava fixes).

Unit tests included.

[1] http://www.eclipse.org/mat/