Allow escaping characters in metadata value

Currently if you add the following metadata to a rule:

rule xyz
@text("it's an \"airplane")
.....
end

if you print the metadata with:

System.out.println(ksession.getKnowledgeBase().getRule("org.sample.myrules", "xyz").getMetaAttribute("text"));

the output is:

"it's an \"airplane"

escaped characters should be handled correctly here.