Add a type named "gradleReference" in framework

It's too complex to dependence AAR in plugin, it need build the plugin
to a android project which include AndroidManifest.xml. This patch
reference the *.gradle in plugin to build.gradle in root project. Below is
a demo.
plugin.xml:
framework src="libs/xwalk_core_library/xwalk.gradle" custom="true" type="gradleReference"

xwalk.gradle:
repositories {
    maven {
        url 'https://download.01.org/crosswalk/releases/crosswalk/android/maven2'
    }
}
dependencies {
    compile 'org.xwalk:xwalk_core_library_beta:9.38.208.4'
}

The build.gradle will add a blow section:
// PLUGIN GRADLE EXTENSIONS START
apply from: 'libs/xwalk_core_library/xwalk.gradle'
// PLUGIN GRADLE EXTENSIONS END