Closes #537: Be lenient to extensions operating on snippet fields

* yasnippet.el (yas--on-protection-overlay-modification): Commit the
snippets being destroyed instead of signalling a protection violation.
Clean up previous change

* yasnippet.el: Update comment.
(yas--protection-violation): Removed.
(yas--on-protection-overlay-modification): Simplified.
(yas--post-command-handler): No longer reference
`yas--protection-violation'
Update README.mdown

Fix misspellings
Add file doc/.nosearch

This keeps `normal-top-level-add-subdirs-to-load-path'
from adding the directory to the `load-path'.
Merge pull request #542 from tarsius/nosearch

Add file doc/.nosearch
Closes #546: backport `yas-initialize' and provide `yas/initialize'

* yasnippet.el (yas-initialize): Renamed from `yas--initialize'.
(yas--backported-syms): Add `yas-initialize'