Change log:
* Send bad message upon entering unsupported request handler
* add c++ wrapper for pho_attrs_foreach
* Phobos_file::get_meta_data returns vector of key-value pairs
* Phobos_file::get_meta_data functions like:
* phobos_getmd_cpp fills the descriptor with attributes for the requested object
* phobos_attrs_foreach_cpp loops over the attrs and returns them to the stream
* Fifo::get_meta_data simply passes on the result of the storage call
* Fifo::get_meta_data returns vector of key-value pairs
* add Getmd request handler
* add getmd_request_handler.h to headers
* request handler has a meta_data member variable
* fill meta_data variable with Phobos attributes
* use repeated calls to ResponseBuilder.header to fill header with Phobos attributes
* send entire message at once
* add support for testing getmd