Make the maximum message size configurable.

This patch gives libcppa users the ability to set the maximum message size. The maximum message size still defaults to 16MB, but can now be adjusted with the function `max_msg_size(size_t)`.

A few points that came to mind whilst adding this feature:

- There also exists a message chunk size parameter, which I purposefully left untouched for now. One could imagine scaling it as function of the maximum message size. This may be too low-level to be part of the public API, but one could imagine adding a similar function for the chunk size of message buffers.

- Why does the copy constructor of `util::buffer` does not inherit the maximum size of the other buffer, but instead uses the default values?

- Although you mentioned to put the implementation of `max_msg_size` in `middleman.cpp`, it may as well fit into `util/buffer.cpp` to be close to the chunk size parameter.