Use find -exec instead of xargs to work around differences between xargs on OSX
and Linux.
Use -b option of gcov - for some reason gcov on Travis can't find .gcno files
otherwise (old version?).
And finally enable config=coverage again.
This expands and replaces the Travis-specific makefile by adding more options and correctly tracking header dependencies.
Also add wchar_t mode test to Travis configuration.
git-svn-id: http://pugixml.googlecode.com/svn/trunk@971 99668b35-9821-0410-8761-19e4c4f06640
