Return null when JLine.readLine() returns null

Hitting ^D causes the shell to continously prompt without an option to close it
other then killing the process.

Previously, when JLine.readLine() returned null, so did
JLineProcessor.readAndParseCommand. This caused the main processor loop break
(line #316) and end the shell. 17bb13a (CRASH-166 : Multiline term input) changed
that and made it return an empty string when ^D caused the end of input.

This patch simply returns null when JLine.readLine() returns null.
Added crash.ssh.idle-timeout property to set custom idle timeout of the underlying SSH server
Added crash.ssh.auth-timeout property to set custom authentication timeout of the underlying SSH server
crash.ssh.idle-timeout and crash.ssh.auth-timeout optional
Added a note in the Groovy section to explain how to switch back to Script repl
CRASH-183 : SSH timeout configuration
Merge remote-tracking branch 'lightpriest/fix-ctrl-d'
Added documentation for SSH server timeouts