fix usage of Cmd in bin_resolver_spec and image_optim_spec
remove changing load path and requiring rspec, create and require spec_helper in all specs

both lib and spec are added to load path when spec is ran using rspec
use stub_const or variables instead of constants to not pollute global context
move with_env helper method inside top describe block of bin resolver helper
use custom matcher instead of extending Fixnum in image optim spec
use test_images instead of TEST_IMAGES in image_optim spec
use temp_copy helper instead of extending ImagePath in image_optim spec
check calls to Tempfile without directly adding methods in image optim spec
use test_images instead of fakes in 'optimize multiple' spec group
test optimize_images_data in optimize multiple spec group
remove group of slow specs ‘bunch’ as it repeats ‘optimize multiple’, part of #60
remove group of slow specs ‘stop optimizing’, part of #60
convert per image specs in group ‘losslessness’ into two specs using parallel optimization, part of #60
fix wording for workers order spec
remove extra level of context in test that each worker can be applied to at least one image
replace slow spec group ‘optimize in place’ with optimize_image! spec, part of #60
replace slow spec group ‘optimize image data’ with optimize_image_data spec, part of #60
remove ‘ignores in place’ spec
merge ‘isolated’ and ‘losslessness’ spec groups into optimize_image spec, remove checking number of created temp files, part of #60
remove ‘ignores data’ spec for broke images

optimize_image_data spec already tests that optimize_image_data uses optimize_image