We already had some IntelliJ IDEA project metadata files in ".idea".
I've revisited and updated those now that I have more experience with
Gradle + IntelliJ IDEA + Git. I think this now represents a better
set of decisions regarding what does and does not belong in version
control.
This commit also extends "README-Gradle.md" with clear instructions on
how to bringup WALA as a top-level IntelliJ IDEA project. The
instructions are of a similar flavor to the Eclipse instructions that
were already present, though the details vary. Most notably, with
IntelliJ IDEA you should *open* WALA as an existing project,
not *import* it as a new Gradle project derived from "build.gradle".
This is exactly the reverse of what one should and shouldn't do for
WALA in Eclipse.