Fix Cucumber reports for cucumber-android

This PR enables standard **Cucumber reports** using the `format` parameter of `CucumberOptions` by
- Not clearing the list of formatters prior to adding AndroidFormatter
- Correctly assigning reporter and formatter before feature execution

Reports will be stored on the device.

Example:
`@CucumberOptions(features = "features", format = {"pretty", "html:/data/data/com.mypackage/cucumber-html", "json:/data/data/com.mypackage/cucumber.json"})`

This will generate an **HTML report** in the cucumber-html directory and a **JSON report**. The `pretty` formatter writes to `System.out`, which will appear as nicely colored output in your logcat, given you use a color aware terminal. See [Cucumber Reports](http://cukes.info/reports.html) for supported formatters/reporters.

After test execution reports can be pulled from the device:
`adb pull /data/data/com.mypackage/cucumber-html`
`adb pull /data/data/com.mypackage/cucumber.json`

You can also write the files to the SD card.

Note that the html reporter requires the cucumber-html jar in the libs directory of your Android test project.