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"
Add recipe for yandex-weather package
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
Add recipe for zenburn-emacs theme
Add recipe for solarized-emacs theme
Remove useless features from yandex-weather
add recipe for auto-complete-pcmp
Clearer package description
add recipe for org-ac
add recipe for plsense-direx
update package list when status changes

Since installation can be asynchronous, refreshing the menu at the end
of el-get-package-menu-execute doesn't help, therefore we update from
el-get-save-package-status instead.
only fetch description when first building menu
replace obsolete :username with :pkgname

since 15d3f8e, :github recipes must use :pkgname user/pkgname instead of
:username user.
don't kill autoload buffer if already visited
Fix whitespace script

Now it loads el-get to pick up custom indentation rules for el-get forms