Custom cache key function per model/collection

I added the option to configure a custom cache key function for each
model/collection, that would ovveride the default cache key function.
fixed lint errors
Merge branch 'custom-key-function' of https://github.com/liorch88/backbone-fetch-cache into liorch88-custom-key-function

Conflicts:
	backbone.fetch-cache.min.js
Elevate var to top of getCacheKey()

 On branch liorch88-custom-key-function
	modified:   backbone.fetch-cache.js
Merge pull request #104 from madglory/liorch88-custom-key-function

Liorch88 custom key function