Added valgrindString() to StackFrame and ParsedError.

Together they produces a string that is as close as possible to the error message produced by Valgrind.
Added id reading pattern to LineMatching.
ErrorParser reads Valgrind id from first Valgrind line and returns it as the third member of the return tuple from ErrorParser.parse.
ParsedError.valgrindString now accepts an id parameter and if provided prefixes every line with '==<id>==', just as Valgrind does.
TestErrorParser receives the Valgrind id from the parser and uses ParsedError.valgrindString(id).

Can be used to compare the result of valgrindString() with the source Valgrind log, but no such comparison is done yet.