CB-3445: Add an initial set of Gradle build scripts

These scripts will build an android project, in debug and release mode.
They also support additional library projects, such as Crosswalk, being
added to libraries.gradle (and settings.gradle). A flag can be set in
libraries.gradle to enable multi-architecture builds.
CB-3445: Add option to build and install with gradle

This gives build/cordova two new command-line arguments: --ant and
--gradle, and will select the build type from those. As a fallback
for the Cordova CLI, the environment variable ANDROID_BUILD can also be
used, set to either "ant" or "gradle".

The default is currently "ant", but it is intended for this to change in
the future.
CB-3445: Add which to checked-in node_modules
CB-3445: android: Copy Gradle wrapper from Android SDK rather than bundling a JAR
Merge branch 'gradle-build-3.x'

This introduces the gradle build system to Cordova-android, behind a flag currently