Add tests for Android plugin detection

Added test codes for detecting Android Plugins (which I wrote in #25).

Android plugin checks Gradle version in `BasePlugin#apply()`
and if it’s not applied on supported versions, `BuildException` will be thrown.

| `gradle-android-plugin` | required on `apply()` |
| ----------------------------------------- | ------------------------------- |
| `< v0.12` | Gradle 1.10+ |
| `>= v0.13` | Gradle 2.1+ |

So I wrote some fake plugins just for testing the target codes.

If `coveralls-gradle-plugin` supports Gradle 2.1 in the future, these test codes should be replaced.

Sorry for dropping the coverage eventually
but I don’t know why Covertura judges that some codes are not covered.
I think they are actually tested.
(I mean, `c.name == name || instanceofWithName(c.superclass, name)`
in `JacocoSourceReportFactory#createTargetSrcDirs()`)

Thanks!