Better documentation of :checksum and :checkout properties

* Explain the difference between :checkout and :checksum
* Document that hg method now supports these properties
document 719264d (even though it isn't merged yet)
Document that checksums may not be abbreviated

As requested in #626.
In hg, "checkout" is properly spelled "update"
yasnippet: only fetch snippets submodule
yasnippet: remove :features and customize hacks

yasnippet now uses load-file-name to come up with a reasonable default
for yas-snippet-dirs.
byte-compile yasnippet.el

It's unclear why compilation was avoided initially[1], but it seems to work
now.

[1]: SVN problems? https://github.com/dimitri/el-get/issues/200
emacs-xkcd recipe
Merge pull request #1601 from ngnmnfn/master

add recipe for emacs-xkcd
Create handlebars-mode.rcp
Add python-environment.rcp
Update jedi.rcp for Jedi.el v0.2.0
Remove features property as it's not needed
chess recipe
Fix Google Calendar build step
Load realgud and not its dependencies from realgud recipe

The realgud recipe is loading (:features) its dependencies instead of itself
even though it has no problems loading its dependencies.  It is currently not
very autoload friendly so load realgud from :features.
add elisp-assorted recipe + dummy recipe files
don't throw error when package menu lacks package

New packages could have become available since the menu was built.
Elite for Emacs recipe
Structured Haskell Mode recipe