Ignore *.tdo files generated by the todonotes package

The [todonotes package](http://www.tex.ac.uk/tex-archive/macros/latex/contrib/todonotes/todonotes.pdf)
generates "tdo" files when you use the \listoftodos command.

LaTeX.gitignore

@@ -29,3 +29,4 @@
 *.toc
 *.vrb
 *.xdy
+*.tdo