Add a getHalfSizeImgTag() convenience method.

This is so we can use the retina technique where you resize a larger image down in browserThis does not touch processing - it's up to the ImageSet and it's Dimensions to have proper compression settings to use this.
getHalfSizeImgTag() comment cleanup
 * it's/its - for shame.
 * retina is not a proper noun.
Merge pull request #72 from isaacgrant/half-size-retina-technique

Add a getHalfSizeImgTag() convenience method.