fixed a bug regarding operations with no preconditions
fixed problems on the XML report
fixed a null pointer that happened when state variables had null values
now considering state variables used in the body of the operation
refactorings
Merge branch 'master' of https://github.com/ernestocid/beta.git

Conflicts:
	br.ufrn.forall.beta/src/tests/integration/XMLReportGenerationTest.java