Allow interfaces to have an optional body, fix #30.

[roipoussiere]

This PR avoid a syntax error when parsing a java8 code with an interface containing a body, like:

public interface Test{ default int foo() {return 0;} }

or

public interface Test{ default void foo() {} }

This 2 codes works now. ref #30

[roipoussiere]

Done.

[c2nes]

Thanks @roipoussiere!