Implement photo size parameter

[mtilda]

This pull request addresses multiple issues around the consistency and control of photo sizing (#117 and #130).

Going forward, users may pass a size parameter that will specify the size of the returned photos. This is achieved by altering the suffix of each img_src in the photos_controller.rb and latest_photos_controller.rb to correspond with the requested size and rover_id. I stored the logic for resizing in photo_helper.rb so it can be shared between both controllers.

Original suffixes (what is stored in the database)

Curiosity	Opportunity	Spirit	Perseverance
.jpg or .JPG	-BR.JPG	-BR.JPG	_1200.jpg

New suffix outputs

Size Parameter Value	Curiosity	Opportunity	Spirit	Perseverance
small	-thm.jpg	-THM.JPG	-THM.JPG	_320.jpg
medium	-br.jpg	-BR.JPG	-BR.JPG	_800.jpg
large or nil	.JPG	.JPG	.JPG	_1200.jpg
full	N/A	N/A	N/A	.png

Note: Some of these suffixes are case sensitive

Other changes

• Move logic for searching photos by params and rover from photos_controller.rb and latest_photos_controller.rb to Photo.search method in models/photo.rb
• Fix non-breaking syntax error in latest_photos_controller.rb (#145)
• Add RSpec test file for latest_photos_controller.rb
• Add RSpec tests for photos_controller.rb and latest_photos_controller.rb with size parameter
• Change how img_src is sequenced in spec/factories.rb, so it does not conflict with tests involving size parameter

Potential problems for users

• When not given a size parameter, photos from Opportunity and Spirit rovers will now be served in their large format; whereas they used to be served as medium.

Notes before merge

I have not tested these changes extensively with Opportunity and Spirit photos, because their scrapers are deprecated. Please test on a full development version of the API with Opportunity and Spirit photos before merging.