more consistent interface for MM event handler
added buffer to the list of announced types
added default impl. for `local_actor::dequeue`
enhanced interface of `util::buffer`

this patch added previously missing member functions to the buffer class
and improves the overall documentation of the class; furthermore, the class
exposes only `void*` pointers to make clear the buffer's content should not
be accessed as string
added missing const qualifier
improved error messages in fd_util
removed redundant code
ignore POLLHUP until there's no more data to read
removed unused code
adapted to new buffer interface
revised middleman layer

this patch removes the possibility to add more than one protocol to a
middleman, because this would most likely break any distributed actor system;
instead, this patch prepares libcppa for allowing user-defined middlemans;
furthermore, this patch establishes a more straightforward parent-child relation
between the middleman and its protocol and added the buffered_writer class
to ease development of asynchronous IO classes