Adds support for Android Studio 3.0.

BREAKING CHANGES:

• dexcount tasks now depend on package tasks, not assemble, due to a change in the variant-output API.
• Instant Run is no longer supported.
• runOnEachAssemble is deprecated and will be removed in a future version; use runOnEachPackage.

Details changes:

• Deprecate runOnEachAssemble configuration property
• Make Android Gradle plugin a compileOnly dependency (#176)
• Disable Instant Run support; incompatible with the new build tools
• Add support for Android Gradle Plugin 3.0.0
• Add class counts (#164)
• Fix zip-file-handle leaks (#160)