Add adapter utility functions

[yumauri]

Addresses issue #35

This PR adds two changes:

1. Separates terms "supported" and "available" for adapters

Previously, if localStorage was disabled by browser's security policies, local adapter just did nothing. like localStorage doesn't exists.

Now, local adapter separates cases, when localStorage is not supported (not exists, for example, in Node environment), then adapter does nothing. And when localStorage is supported (exists), but disabled by security policies — adapter will trigger errors on each read/write operation. Errors could be handled by fail event, as usual.

This is breaking change.

2. Adds either utility function

Given two adapters, this function will return first one, either second one, if first one is "no-op" adapter, meaning "not supported" / "does nothing". For example, nil adapter is no-op adapter. local adapter in Node environment is no-op adapter.