Convert `EUAPI` into a singleton
EUAPI should only ever be network-activated on a Multisite install
Make `EUAPI::check()` a static method
Some stricter type checking
Better parameter name for the `upgrader_pre_install` callback
Small tweaks
Move the default `type` config option into the `EUAPI_Handler` class
Add support for an upgrade notice
Some extra type checking
Doc corrections
Important bugfix
Correct the company name
Docblock additions
Bit of alignment
Handler type validation
Change `raw.github.com` to `raw.githubusercontent.com`.

See https://developer.github.com/changes/2014-04-25-user-content-security/ for more details.
Introduce another abstract handler for plugins and themes where the plugin or theme updates are fetched simply by reading the plugin or theme files from a URL (as opposed to communicating with an update API).
Handler arguments aren't optional
Document the `$config` args for handlers
Correct the update destination for themes. Fixes #2.