Update mongodb formula

 * On 10.9, use libc++ (not libstdc++) for devel and HEAD builds
 * Pass --full flag to install headers and library also.
 * Add --use-system-boost, to avoid linking issues with the created library.

Questions: 
 * Is passing --full flag unconditionally acceptable? What is the policy on installing development files? At the moment for ROS there is a conflicting formula https://github.com/ros/homebrew-hydro/blob/master/mongodb-dev.rb, but I would prefer to combine the two if possible.
 * Should --use-system-boost be conditioned on an optional dependency on boost? How does this work?