Issues/427 add edit image meta *** DO NOT MERGE ***

First pass (and work in progress) for adding the ability to edit image meta data, e.g. link, caption, etc.   Ready for your feedback!! 

### Points of interest!
- Adds two new JS libraries: underscore.js and shortcode.js (from core WordPress)
- Image parsing logic was borrowed and modified from wp core in order to be as consistent as possible with existing user expectations
- Adds a WPImageMeta convenience class as a simple holder for meta values. 
- Adds a simple modal to demo the edit feature. 

### TODOs
- There is some data sanitation and safety logic that needs to be implemented.
- There are some slight differences in the logic that parses out the image, and the logic that rebuilds it. These need to be made uniform. 
- Some meta values can be derived from other meta values so the total number can likely be reduced (by at least 2)

### Questions:
What are your thoughts on the best approach to the demo form? Its as simple as possible right now. 

### Misc:
I'm sure I'm forgetting something. :) 

@SergioEstevao @bummytime @diegoreymendez 



I'd prefer to keep the methods borrowed from WordPress core as close to intact as possible.  Changes to the methods in core would likely be valid changes to the methods here.  There are some modifications from original (that i need to better document!) that would need to be taken into account, but splitting the methods up would make it a bit more of a challenge.