Fixed definitions of the OT_DIR and OT_URL constants for the OT theme mode

On win platform it was impossible to put the option-tree pugin into the subdirectory of the theme. I am using bower as a dependency manager and have the option tree in `[...]/my-theme/bower_components/option-tree/` folder. The `basedir( __FILE__)` on win returns the backslashes `\` instead of slashes `/` so the whole explode thing did not work.

What I did is that I simplified getting the theme directory name - `get_template()` does that and performed the string replacement on `dirname` function to replace all `\` with `/` prior exploding.