Added prlx-slides custom post type and support to look for a static-slider.php template in theme directory.

Added a custom post type called "prlx-slides" this is solely intended
for use with the parallax slider and was added to simplfy the creation
of slides separately from other post types on a installation.
wp-parallax-content-slider.php
- Added a new option "post_type" with the default value "prlx-slides" -
$prlx_slider_post_type.
- Modified the $args variable used to build the slider starting on line
267.
-- If the selected slide mode is equal to "prlx" the post type is set to
the newly added option $prlx_slider_post_type.
-- Otherwise it defaults back to $typ.
- Added a third option to slider mode called Slide builder which used
the prlx-slides custom post type.
- Added support to include static-slider.php templates in the theme
directory.
-- Starting line 357 if the slider mode static is chosen another check
is performed to see if a "static-slider.php" template exist in the
currently activated theme otherwise it will default to the
static-slides-sample.php included in the plugin.
- Incremented version number from 0.9.6 to 0.9.7.
- Removed whitespace after closing php tag.
content_slides.php
- This houses the code for the prlx post type.
- There are 2 custom meta fields used these are for images and url.
-- Both choosing and image and/or a url re-use the functionality found
in wordpress for example selecting an image opens the media library.
-- Selecting a url allows the user to choose from a drop down list of
pages on the current site. (this could be modified to include post and
other post types if required).