Allow policy actions to be defined via class methods only

[chrisalley]

Following the discussion in #41, it would be simpler to be able to define actions without explicitly defining an actions map, e.g.

class PostPolicy extends Policy {
  user: AuthorisableUser
  record: AuthorisablePost

  constructor(user: AuthorisableUser, record: AuthorisablePost) {
    super(user, record)
  }

  view(): boolean {
    return true
  }

  publish(): boolean {
    return this.user.id === this.record.userId
  }

  destroy(): boolean {
    return this.user.isAdmin
  }
}

[johno]

100% agree there!

[chrisalley]

Solved in #48 and #49.