Remove va-auth-adapter and move to its own npm pacakge

[sgobotta]

Regarding #11 , this would take us a step closer to introduce an auth provider in vuex-crud.

A very basic implementation of an auth provider using axios helped me reach an idea of how this funtionality should work. When Vue Admin supports even more auth action types, an adapter would be able to take advantage of them and trigger different services in the Vue-Admin store.
When this package becomes functional we would be ready to provide a nice and clean implementation for the vuex-crud library.

Checkpoints

• Publish a first version functional version of the adapter in npm
• Publish a Vue Admin version that exposes the auth action types
• Test the va-auth-axios-adapter dependency in Vue-Admin
• Run the e2e tests, they must pass.
• Run the unit tests, they must pass.