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
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
Merge pull request #1 from opentable/add-tests

Add Unit and Functional Tests