add $ tilde operator for dynamic index entry

a way to change the value of items in the right array element, where element to match is based on some condition.

perhaps this is not the most generalized form, nor the write name - to be discussed.

see
http://groups.google.com/group/mongodb-user/browse_thread/thread/58951cdf45c57e38/a37bfe469292b9fc?hl=en&lnk=gst&q=obtain+index#a37bfe469292b9fc