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
add recipe for org-linkany
Merge pull request #1592 from aki2o/org-linkany

add recipe for org-linkany
Add recipe for edebug-x
Add recipe for pelican-mode
Merge pull request #1593 from qdot/edebugx-pelican

Recipes for edebug-x and pelican mode
remove unused variables

They're unused since 4b85baa9a11b4a51f92a2bc042d177f79b269b92
el-get-init-and-install: handle new :depends

If a package to be init'd has new dependencies, simply install them
rather than throwing an error about missing packages. A warning about
changing non-whitelisted properties is still issued.
Merge pull request #1594 from npostavs/new-dep

Handle change in :depends
`f' depends on `s' and `dash'
Merge pull request #1595 from zonuexe/add_f_dependency

`f' depends on `s' and `dash'
el-get-set-info-path: make safe if package removed

Use an eval-after-load form that won't throw an error, regardless of
removed packages.
don't use called-interactively-p

Replace it with always t interactive spec, as suggested by its
docstring.
Merge pull request #1597 from npostavs/no-interactive-p

don't use called-interactively-p