Implement IndexDriver Load and Delete for pilosa index

[ajnavarro]

Create the pilosa IndexDriver implementation:

// IndexDriver manages the coordination between the indexes and their
// representation in disk.
type IndexDriver interface {
	// ID returns the unique name of the driver.
	ID() string
	// Create a new index.
	Create(path, table, db, id string, exprs []Expression) (Index, error)
	// Load the index at the given path.
	Load(path string) (Index, error)
	// Save the given index at the given path.
	Save(ctx context.Context, path string, index Index, iter IndexKeyValueIter) error
	// Delete the index with the given path.
	Delete(path string, index Index) error
}

And implement Load and Delete Methods.

[erizocosmico]

This depends on #174

[kuba--]

It's done by: #205