Better generalize base client package

[Ulexus]

It would be useful to generalize an interface for an apiban store so as to facilitate more clients and intermediate caching and storage.

Proposing the following definitions:

// Store defines and interface for storing and retrieving entries in the APIBan database, local or remote
type Store interface {

	// Add inserts the given Listing into the store.  Listing may be sparse, requiring only the IP.  Returned value will be fully populated.
	Add(ip *Listing) (*Listing, error)

	// Exists checks to see whether the given IP matches a Listing in the store, returning the first matching Listing, if one exists.
	Exists(ip net.IP) (*Listing, error)

	// List retrieves the contents of the store
	List() ([]*Listing, error)

	// ListFromTime retrieves the contents of the store from the given timestamp
	ListFromTime(t time.Time) ([]*Listing, error)

	// Remove deletes the given Listing from the store
	Remove(ip *Listing) error

	// Reset empties the store
	Reset() error
}

// Listing is an individually-listed IP address or subnet
type Listing struct {

	// ID is the unique identifier for this Listing
	ID string

	// Timestamp is the time at which this Listing was added to the apiban.org database
	Timestamp time.Time

	// IP is the IP address or IP network which is in the apiban.org database
	IP net.IPNet
}

This will set us up to have a pluggable system of stores by which we can make an extensible, pluggable set of client functions.