Reorganize autoload file
Replace s:clear_visual_marks() with delmarks < >
Ensure all visual mark setting is done via s:set_visual_marks
Replace setpos("'<") calls in plugin script
Add backwards compatible s:set_visual_marks()

Vim patch 7.3.590:

    updated for version 7.3.590
    Problem:    The '< and '> marks cannot be set directly.
    Solution:   Allow setting '< and '>. (Christian Brabandt)

This change was committed less than a year ago, leaving most vendor
installations of vim unable to run vim-sexp.

Closes #1
Handle current visual state in s:set_visual_marks()