Implement targetSdkVersion and maxSdkVersion, plus version code names.

This adds new attributes for specifying a targetSdkVersion and maxSdkVersion.
There is a new ApplicationInfo flag that is set if the application has set
its targetSdkVersion to the current platform or later.  Also you can now
use a string for minSdkVersion and targetSdkVerion, to indicate you are
building against a development tree instead of an official platform.