Adding support to multiple galleries

[ddomingues]

This change provides:

• Flexibility to select the element with a another selector different of the id. Providing you support to multiples galleries.
• Adding galleryItem option to choose another item type
• All elements added to gallery, now inherit the onclick event 😄

I needed of these features, because I have an implementation that has a gallery that should change when I choose a different product, i.e., an e-commerce product that have different colors and It has different pictures to each one

[igorlino]

@ddomingues Hi there, FYI, I have re-created the elevatezoom into EZPlus, an up-to-date project at:
https://github.com/igorlino/elevatezoom-plus
Although it has already many build changes, as of today it has nearly the same JS code as KyleMacey' fork (which had the most bug-fixes from all other elevatezoom forks. Thanks for your efforts)
Happy to receive PRs.