Add default fixtures for BOTR related media.
Merge pull request #75 from isaacgrant/botr-media-fixtures

Botr media fixtures
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.
Abstract image upload tool.

Sites need not use the component, but it can be useful to have the common base class for all image uploads.
Add file type methods to mirror SiteAttachment. These are all needed for the common image upload tool.