Added support for un/reloading the Spread library in src/reloading.lisp

* src/reloading.lisp: new file; contains support for unloading and
  reloading the Spread library
* src/package.lisp (package network.spread): added exported symbols
  unload-spread-library, reload-spread-library and
  enable-reload-spread-library
* network.spread.asd (system network.spread): added file
  src/reloading.lisp
Improved documentation strings in src/{connection,package}.lisp

* src/connection.lisp (connection::join-hook): improved documentation
  string
  (connection::leave-hook): likewise
* src/package.lisp (package network.spread): extended documentation
  string