Drop all usage of URI to make tree-shaking urijs dependency possible

[Shaptic]

What

Replaces usage of URI (from urijs) with native URL.

There is still a reliance on urijs's URITemplate API for building URLs from Horizon's _links responses, but this should still lower bundle size as its an explicit, standalone import.

Why

Reducing bundle size, see #824.

Assets: 
  stellar-sdk.js (1.83 MiB)
  stellar-sdk.min.js (685 KiB)

Known Limitations

Is this worth it? Tests pass, but this might break in subtle ways.

[aristidesstaffieri]

lgtm, I suspect we can drop uri.js entirely in favor of native URL + template strings but that's another topic.