Closes #546: backport `yas-initialize' and provide `yas/initialize'

* yasnippet.el (yas-initialize): Renamed from `yas--initialize'.
(yas--backported-syms): Add `yas-initialize'
Merge branch 'master' of enzochiau.github.com:enzochiau/yasnippet
Merge branch 'master' into ec_dev