SERVER-9526 a more elegant solution to printout of BinData in log messages

This commit changes the behaviour of BSONElement::toString() to print BinData in the same way as the other element types. It puts a cap on the lenght of the binary data in the same way as for string. Using full == true overrides the cap in the same way.

Signed-off-by: Greg Studer <greg@10gen.com>