jline/jansi fixes for windows. Fixes #763, fixes #562.

The startup script should set sbt.cygwin=true if running from cygwin.
This will set the terminal type properly for JLine if not already set.
If sbt.cygwin=false or unset and os.name includes "windows", JAnsi is
downloaded by the launcher and installed on standard out/err.

The value for jline.terminal is transformed from explicit jline.X to
the basic types "windows", "unix", or "none".  Now that sbt uses JLine
2.0, these types are understood by both sbt's JLine and Scala's.
Older Scala versions shaded the classes but not the terminal property
so both couldn't be configured with a class name at the same time.