Preliminary support for GNUstep+libobjc2.

All unit tests pass.
Ubuntu setup script.
Fixed Darwin build.
enabled a few more tests for GNUstep build.
minor edits to Ubuntu.sh setup script.
minor change to install location for Nu library .nu files
minor ifdef simplifications
Merge branch 'gnustep-libobjc2' of github.com:timburks/nu into gnustep-libobjc2
more minor cleanup, reenabling isatty check for linux
Merge branch 'gnustep-libobjc2' of github.com:timburks/nu into gnustep-libobjc2
isatty simplification
restored "def" as a synonym for "function".
Merge branch 'master' of github.com:timburks/nu into gnustep-libobjc2
removing unneeded debug code
run apt-get update at start of Ubuntu installation script.
added sortUsingBlock: to NSMutableArray
Merge branch 'master' of github.com:timburks/nu into gnustep-libobjc2
Merge branch 'master' of github.com:timburks/nu
Merge branch 'gnustep-libobjc2' of github.com:timburks/nu