Optional msg argument to leiningen.core.main/exit. Use it for abort.

- New signature: leiningen.core.main/exit [exit-code & msg]
- msg used when *exit-process?* is false as the Exception message
send stack trace through to the nREPL client
Don't show the "Suppressed exit" message when exit is called
Initial happy-path unit tests for the clean task. Relates to #1458
Partial implementation of fix for :clean-targets sanity checking (issue #1458)
Fix for #1458: clean-targets sanity checks.
Removed (run-tests) call that was being used for interactive REPL testing
Merge remote-tracking branch 'upstream/master'
Making new methods private for issue #1458
Support for overriding :clean-targets sanity checking (issue #1458).
Also made unit tests safer and faster using with-redefs to mock out
calls to delete-file-recursively.
Better error messages when a path is being protected.
Merge pull request #1461 from cpmcdaniel/master

Fix for #1458: clean-targets sanity checks.
Merge pull request #1440 from benwbooth/master

Show uncaught exceptions when using grenchman, don't display "Suppressed exit" when calling exit
Fix test failures due to missing artifacts.

Somehow a dependency of nodisassemble disappeared from Maven Central.
Fix tests for task arity mismatch abort messages.
Merge remote-tracking branch 'laurentpetit/master'

Conflicts:
	leiningen-core/src/leiningen/core/main.clj
Make repl tests resilient to having .nrepl-port file in pwd.