#121 Initial version of shortcode

TODO:
- Linking to other posts by providing post title not working yet
- Properly integrate short code code into plugin structure
Added images for premium content

Added some default images for premium content.
Added "Shortcode" to the Readme

Added the "shortcode to highlight premium content" to the readme.
#121 Prepared retina / non-retina support
Added non-retina preview images

Added non-retina versions of the default preview images for premium content
#121 Added documentation for shortcode
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode
Merge branch 'feature/121_premium_downloads_shortcode' of github.com:laterpay/laterpay-wordpress-plugin into feature/121_premium_downloads_shortcode
#121 simpler but working retina detection
#121 Renamed function from renderPremiumDownloadLink to renderPremiumDownloadBox
Updated default preview images

Updated the default preview images (minor graphic changes, added shadows)
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode

Conflicts:
	laterpay/README.txt
#121 Fixed stoooopid get_post_by_title function call; renamed images and references to them
#121 Some clean-up for shortcode code
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode

Conflicts:
	laterpay/README.txt
Merge branch 'refs/heads/master' into feature/121_premium_downloads_shortcode