Fix missing RuntimeException class error

Signed-off-by: Alexander Cogneau <alexander.cogneau@hotmail.be>
Fix double \\
add plugins to save() method
add sharpen plugin

add sharpen plugin with imageconvolution method
add sharpen documentation
* added new example pages
 * PHPThumb::pad()
 * PHPThumb::imageFilter()
 * PHPThumb::adaptiveResizeQuadrant()
* Removed broken links in README.md
Merge branch '2.0.2' of https://github.com/remiheens/PHPThumb
Merge https://github.com/AlexCogn/PHPThumb