Generate an error when making a path string from paths containing the separator. Fixes #1038.

This is an attempt to provide a decent error message in some cases.  However, paths that include
the Java path separator character are just fundamentally problematic and aren't always going to
be cleanly detected.
add flyway-sbt plugin to community plugins
API documentation for TaskStreams.
Merge branch '0.13' of https://github.com/sbt/sbt into 0.13