Feature #31, revoke method ldap

[guisithos]

Added a new CachedLDAP type that wraps the existing LDAP strategy and maintains access to the cache

type CachedLDAP struct {
    auth.Strategy
    cache auth.Cache
}

Then add the Revoke method:

func (c *CachedLDAP) Revoke(hash string) error {
    c.cache.Delete(hash)
    return nil
}

And per last, modified the NewCachde:

func NewCached(cfg *Config, c auth.Cache, opts ...auth.Option) auth.Strategy {
    fn := GetAuthenticateFunc(cfg, opts...)
    strategy := basic.NewCached(fn, c, opts...)
    return &CachedLDAP{
        Strategy: strategy,
        cache:    c,
    }
}

This should allow immediate invalidation of cached authentication without impacting any existing implementations.