Merge pull request #1562 from 153957/patch-1

Ignore files created by \tikzexternalize

When using TikZ & PGF with:
\usepackage{tikz} \usetikzlibrary{external} \tikzexternalize
It creates .dpth and .md5 files for the externalized tikzpictures.

See these pages in section 50 Externalization Library:
p.617 - 50.4.1 Support for Labels and References In External Files
p.621 - /tikz/external/up to date check
In the PGF manual:
http://mirrors.ctan.org/graphics/pgf/base/doc/pgfmanual.pdf