Ubuntu 18.04 support, gnustep-base mainline

This release updates the supported/tested version of Ubuntu to 18.04 and depends on a more recent version of the GNUstep base library.

(version) update

Very minor update to synchronize the (version) operator with the current version.

Xcode 8.3 and Ubuntu 14.04

This release adds Travis CI of Linux builds on Ubuntu 14.04. macOS builds are verified with Xcode 8.3. Future commits must maintain compatibility with both macOS and Linux.

Run the ubuntu.sh script in a bare Ubuntu instance to install Linux dependencies. libobjc2, gnustep-make, and gnustep-base are vendored from forks in http://github.com/nulang.

Restructuring

This release restructures Objective-C source code into separate files per class and category.

Last amalgmation

Final snapshot of "amalgamated Nu", which collected all source into two files (a header and a .m file) for easier embedding.