Add failing test for missing commas in @external statement.

gwt-dnd was missing a comma in an @external statement and was
not failing the build due to that.

fixes #78

Move @external rules to top level since GSS expects them there.

fixes #89

Conflicts:
	src/main/java/com/google/gwt/resources/converter/Css2Gss.java
	src/main/java/com/google/gwt/resources/converter/GssGenerationVisitor.java
	src/test/java/com/google/gwt/resources/converter/Css2GssTest.java

Conflicts:
	src/test/java/com/google/gwt/resources/converter/Css2GssTest.java

Conflicts:
	src/test/java/com/google/gwt/resources/converter/Css2GssTest.java

Conflicts:
	src/test/java/com/google/gwt/resources/converter/Css2GssTest.java
Merge pull request #94 from dankurka/fix_at_external_being_used_in_otherruls

Move @external rules to top level since GSS expects them there.