Switch to the Gradle build system.

See issue #1 for details.

This commit only changes the core, desktop, and Android projects. The
HTML and iOS projects will fail to build until they are updated to use
Gradle as well.
Forgot to include Gradle wrapper jar.

Oops.
Fix readme.
Delete unnecessary leftover files.
Use latest Android SDK for Travis CI builds.
Force Gradle to use plain terminal in Travis CI builds.
Refactor Travis CI configuration to support other CI services.
Tell Gradle to ignore the exit value when running the desktop build.
Merge pull request #1 from EbenezerEdelman/issue/1/gradle-build-system

Switch to the Gradle build system.