Don't override global $post

Because the `woothemes_testimonials()` function declares the global `$post` object and then uses the variable name `$post` when [looping over testimonials](https://github.com/thenbrent/testimonials/blob/ba8655d335dd5872e0b7fab64549f9c2e55a0147/woothemes-testimonials-template.php#L91), it overrides the global `$post` object. 

This means anything that needs to access the current `$post` after the `[woothemes_testimonials]` shortcode will actually be left only with access the last testimonial, not the current post object.

I didn't see anywhere the global `$post` object was actually used in the function, and I suspect it's not intentional to override the global post.