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.
Add file type methods to mirror SiteAttachment. These are all needed for the common image upload tool.
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.
Abstract base class to delete site images.
Add some default styles for the image upload page when there is an existing image.

These are recklessly stolen from Store's category details page.
Set the valid mime and human types on the dimension upload widgets.
Better dimension notes.
 * real mulitplication symbol
 * better handling of when only width or height is set including dropping the plural on "Maximum Dimensions..." and explicitly stating which dimension the maximum size applies to.
Better error message from @nburka.
Add getDimensionUploadWidget method, and use it whenever accessing the dimension uploads.
Drop an IE6 style copied from very old code.
Most of the confirmation message code now exists in AdminObjectDelete.
Hide the mime types note from the dimension upload widgets.

The note is already displayed on the automatic upload, and it gets overwhelming when displayed on every widget.
Take advantage of replacing object support added to AdminObjectEdit

Requires https://github.com/silverorange/admin/pull/27
Get rid of css setting the width of the displayed image - it never made any sense.
Merge pull request #76 from isaacgrant/image-admin-component

Image admin component