Make ids unique in html code for callbacks.

I found that functions/macros and callbacks share the same id when you want to link to these ids in the docs.
for example:
http://elixir-lang.org/docs/stable/elixir/Dict.html#delete/2
both delete/2 function and delete/2 callback point to the same link.
this is wrong as well, when visiting the links on the left bar.

This solves the problem by prefixing callbacks with "c:"  (same is it is used for prefixing types with "t:")

So callback ids will be like:
http://elixir-lang.org/docs/stable/elixir/Dict.html#c:delete/2