remove dependency on stdlib package

This commit removes the dependency on the Stdlib package so that the Dom package can be used completely standalone. It does so by extending DOMXpath to wrap each raised error with an ErrorException with logic similar to that used in Stdlib ErrorHandler.
use end() instead of convoluted count()
add back vendor/.gitignore, thanks @samsonasik
Merge pull request #5865 from pmjones/develop

remove dependency on stdlib package
[#5865] Fix filesystem filename case

- s/DOMXpath/DOMXPath/ so that filename and class name match (fixes
  failure in Zend\Dom\Document\Query test)
[#5865] Added note to README.md
Merge branch 'feature/5865' into develop

Close #5865