Rename arguments in el-get-fetcher-registry

Rename "type" to "fetcher" and "operation" to "property", to better
reflect the current nature of these arguments.
Add function "el-get-package-dependencies"
Add and use "el-get-recipe-get-list"
Allow fetchers to specify dependencies

This is important for e.g. elpa type which requires package.el to be
installed. (Although that example may be obsolete since I am targeting
Emacs 24 & up.)
Add support for "init" forms in recipes

The three phases are ":init" ":after-init", and ":after-load".
Add el-get-init.el