Add defcallback snippet.
Add defmacrocallback snippet and addition to syntax list for highlighting.
Add defdelegate snippet.
Add snippet for defexception, with replaceable :message atom as default.
Add snippet for defoverridable with placehoders for a keyword list.
Add snippet for defprotocol.
Add defstruct snippet with omitted brackets, since this seems to be the popular norm.
Merge pull request #24 from ybakos/def_snippets

Add snippets for various defs