Added [listing_search] shortcode and fixed widget

Added [listing_search] shortcode and common function
Template filter fix and [listing_search]

New: Added [listing_search] shortcode
Tweak: Updated Listing search widget for backward compatability allows
removal of search options
Fix: Ability to add own templates for single and archive views with
action
Merge pull request #18 from easypropertylistings/1.2-search-widget

1.2 search widget