Only avoid collision if it's the same file

@chriseth had suggested that it would be better if contracts were
referenced in a file:contract notation, and that we output .bin files
that prepend original path names if necessary to avoid a collision.

This commit is mostly a draft; it still needs to be run through the test
suite.

libsolidity/ast/AST.cpp

@@ -190,6 +190,12 @@ void ContractDefinition::setUserDocumentation(Json::Value const& _userDocumentat
 }
 
 
+std::string ContractDefinition::fullyQualifiedName() const
+{
+	std::string qualifiedName = *(location().sourceName) + ":" + name();
+	return qualifiedName;
+}
+
 vector<Declaration const*> const& ContractDefinition::inheritableMembers() const
 {
 	if (!m_inheritableMembers)