Add manipulation calls

2.4.0 - Add some item manipulation

#42 : Add option to get list of keys
#40: Allow 'Touch' on items without other operation

// Touch resets the TTL of the key when it exists, returns ErrNotFound if the key is not present.
func (cache *Cache) Touch(key string) error 

// GetKeys returns all keys of items in the cache. Returns nil when the cache has been closed.
func (cache *Cache) GetKeys() []string