JST templates have directory separator normalised

On windows, JST templates with nested directories will have backslashes
in the name. All / or \ characters in the JST template name will be
replaced with the configured separator, or default to /
common_path_prefix accepts / and \ as separators

The common_path_prefix implementation only used os.path.sep for
splitting the string, which doesn't handle filenames with / separators
on windows causing tests to fail.
Added test cases for JST directories
Fixed new JST tests: should pass on not-windows too

Using os.path.normpath so that test case will only see backslashes in
path on Windows.
"Merge pull request #370 from marcellarius/master\n\nFixes for JST filter with Windows filenames."