Don't show placeholders for images on the reader detail.

Starting width and height is 1px instead of 0px to avoid graphics context warnings.
Clarifies variable name.
Fixes an issue where not specifying a success block for a ReaderMediaQueueItem would prevent the downloaded image from showing.
Removes obsolete placholder specific code.
Merge pull request #982 from wordpress-mobile/issues/981-remove-placeholders

Don't show placeholders for images on the reader detail.