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
Merge pull request #1577 from npostavs/yasnippet-simplify

Yasnippet: avoid yasmate submodule, and remove hacks
* recipes/virtualenv.rcp:  Remove deprecated package fixes #1588
* recipes/virtualenvwrapper.rcp: new package.
* recipes/pubmode.rcp: new recipe
* recipes/temp-buffer-browse.rcp: new recipe.
* recipes/writer-typewriter.rcp: new package.
* recipes/prolog-el.rcp: fix url.  fixes #1521
Remove yas-jit since yasnippet has builtin jit feature.
add projectile-rails
Merge pull request #1626 from shishi/add_projectile_rails

add projectile-rails
Merge pull request #1624 from xiaohanyu/remove-yas-jit

Remove yas-jit since yasnippet has builtin jit feature.
Add recipe for window-margin mode
add depends for projectile-rails