Adding option for default testers

Default testers can be specified in tiapp.xml which are then automatically invited to the latest build. This removes the need to invite testers in the web dashboard for new apps.

New tiapp.xml property:
```
<property name="installr.default_testers">someemail@domain.com, other@email.com</property>
````
Update README.md
Merge pull request #2 from sschueller/patch-1

Adding option for default testers