The Loader is able to return a list of external variable.
External variables can be externally set
External variable not typed
Repackaging
Add contracts and precondition to external variable class.
Model refactoring
Package refactoring
GprParser and GprLexer ignored by git
Removed unused method.
Merge branch 'master' of https://github.com/cdonnat/gpr4j.git

Conflicts:
	org.gpr4j.test/src/org/gpr4j/core/internal/test/BuilderTest.java
	org.gpr4j.test/src/org/gpr4j/core/internal/test/LoaderTest.java