Add Basic Maven Deployment Support

Build the AAR and all supporting Maven Central ready materials in the
build directory.

- The AAR will be in ex.
build/artifacts/1.0.1/com/paymentkit/paymentkit-android/1.0.1/paymentkit
-android-1.0.1.aar
Styleability Tweaks

Small tweaks to facilitate users of the AAR overriding styles,
dimensions, colors, etc.

- Also wrap some init code in FieldHolder and CardNumHolder in
View.isInEditMode() blocks so that users of the AAR don’t get errors in
the Android Studio editor preview.
- Avoid potential exception when validating an empty card number field
in FieldHolder.
- Set the execute bit on gradlew.
Update Maven Deployment Support

Use patched maven plugin (from
https://github.com/dcendents/android-maven-plugin) that works around
issues caused by the Android Gradle plugin overriding the ‘install’
task which Maven uses.

- Use global artifact version number in gradle.properties that can be
bumped up and checked in when new releases are ready.
Add Unit and Functional Tests

Add a few basic unit and functional tests. The functional tests use a
custom Activity and resource from the test sourceSet.

- Note: The Gradle Plugin v 0.13.0 and later allow merging test
manifests. I found that v 0.13.2 and 0.13.3 throw cryptic errors when
processing the manifests. Gradle Plugin v 0.13.+ requires Gradle 2.1
Rebasing branch 'add-tests' of https://github.com/opentable/PaymentKit-Droid from 'master'

Conflicts:
	build.gradle