prepare for travis-ci.org continuous integration

Travis CI is a free hosted continuous integration platform for
open-source projects.  It allows automated testing for GitHub-hosted
projects.

This commit adds a corresponding `.travis.yml` configuration file.

.travis.yml

+language: cpp
+
+compiler:
+ - clang
+ - gcc
+
+env:
+ - config=debug64   config_suffix=debug_x64_gmake
+ - config=release64 config_suffix=release_x64_gmake
+
+before_install:
+  - sudo add-apt-repository -y ppa:codegear/release
+  - sudo apt-get update -qq
+  - sudo apt-get install -y premake4
+
+install: true
+
+before_script:
+  - cd build 
+  - premake4 'gmake'
+  - cd "${TRAVIS_BUILD_DIR}"
+
+script:
+  - make -C build/gmake -f test.make unittest
+  - cd bin
+  - ./unittest_${config_suffix}