Add pending test case for #958 (incremental compiler bug)

The ticket contains detailed description of the problem. The test case
just shows that if we set `incOptions := sbt.inc.IncOptions.Default`
then the incremental compiler doesn't recover from compiler errors
properly.
Docs: fix typo.  Fixes #951.
Merge remote-tracking branch 'gkossakowski/error-in-invalidated' into 0.13
Add in the ability to launch non xsbti.AppMain classes.

* Deprecate old mainClass method on appProvider
* Create entryPoint method which represnets class used to launch instead.
* Create PlainApplication to wrap static `main` methods.  Can return
  either Int, Exit or Unit.
* Detect supported 'plain' classes via reflection.
* Add new unit tests appropriate to the feature.
Allow specifying sbt.boot.properties as a URI

This allows straight forward system independent generation of the
property, since URIs are well specified and only support one file
separator.