Add initial `Follow` implementation

This doesn’t offer a whole lot other than wrappers for calling the REST
API functions for following blog sites.

Honestly, this may not even do anything, but it’s there to test with to
allow me to explore the implementation options.
Remake dist/wpcom.js
Add Follow instance to Site
Add initial `Follow` documentation
dist/wpcom.js recompile
Add stub test for Follow
dist/wpcom.js recompile
Follow: Expose `site.follow()`
Test with exposing function
Undo previous commit exposing test followme()
Expose Follow class in module.exports
Fix typo `side_id` > `site_id`
Fix API call typo `follows/delete` > `follows/mine/delete`
Expose Follow in main `wpcom` index.js
Added missing semicolons
Update code formatting
Move Follow under Site from top-level

Previously, `Follow()` lived as its own top-level class. However, it
belongs as a child of `Site()`.

It should be called as `Site().follow()` instead of `Follow()` directly.
Site->Follow typo: replace `_sid` with `_id`

`_sid` is the wrong variable, and it was incorrectly called for when
moving `Follow()` with the previous commits.
Follow: Rename functions to match style and pass query

This commit brings the naming conventions more in line with the other
modules.

It also passes the optional `query` parameter, which was simply passed
as `null` before.
Add missing semicolon;