maint: move README (still empty) to the root of the repository.
maint: restructure repository in preparation for mavenization.

* move tango/ into src/main/java/tango/, and plugins.config into
src/main/resources/ as first step into making use of Maven for build
system.
maint: add a gitignore file.
Add pom.xml for automatic build with Maven.
maint: fill in README file with small intro and list of non-mvn dependencies.