Fixed StringData repr() to support newlines and quotes.

Since this now uses Python's own internal repr() implementation, it should support any combination of characters at all.

Fixes ImageEngine/gaffer#837
Fixed VectorDataTest to account for StringData repr change.

Python defaults to single quotes where possible, but we were using double quotes previously. There is no change in the result of an eval( repr() ) expression.
Merge pull request #268 from johnhaddon/stringDataRepr

Fixed StringData repr() to support newlines and quotes.