Work around glob() returning non-existing files.

Because vim's glob() function uses the current shell to do the actual
globbing, accepted patterns depend on the used shell (POSIX vs. zsh vs.
bash vs. ...). On my system, this leads() to glob() returning
non-existing files, so work around that by filtering the list returned
by glob() using filereadable().
Added link to Arduino snippets project in README
Added protection against regular expressions in trigger.

For example, if triggerR contains a tilde, one can get a
'No previous substitute regular expression' error
Fixed custom backwards trigger key.

Usage of the variable g:snips_trigger_key_backwards caused an extra '>'
to be added to the resulting backwards-binding, regardless of whether it
was needed.
Merge branch 'master' of https://github.com/apuignav/vim-snipmate

* 'master' of https://github.com/apuignav/vim-snipmate:
  Added protection against regular expressions in trigger.
Fix a few grammatical errors in README

Fix a few grammatical errors.  Nothing major.
Merge pull request #106 from codeinthehole/patch-1

Fix a few grammatical errors in README
Merge pull request #93 from sudar/arduino

Added link to Arduino snippets project in README
Merge pull request #99 from apuignav/master

Added protection on DefaultPool for regular expressions in triggers
Merge pull request #91 from Tblue/master

Work around vim's glob() function erroneously returning non-existing files
Merge pull request #101 from linduxed/custom-backwards-fix

Custom backwards key gets extra bracket
Fixed unexpected errors related to glob method
Bring in line with what the README says
Some editing.

The VAM install instructions aren't code.
Add zedr's Zope snippet repo.
Mark the "Why forking" section as a blockquote instead of a code block.
Merge pull request #113 from jean/master

Minor cleanup, and referenced @zedr's repo.
Merge pull request #112 from mihaifm/master

Unexpected errors related to glob method, when no snippet is found
guard must start at first column
fix pull/78
correct statement, add neosnippets to list of related work. I haven't tried it yet
add me as maintainer along with Garbas too, cause its me fixing bugs
along with him and others
bcd592ce5e fixed the ~bug introducing a new one, ctrl-r<tab> broke.
The real fix is to escape ~ rather than disallowing regular expression
matching