Add support for search API

[drborges]

type Tag struct {
  db.Model     `search:"tags" db:"Tags" `
  Name  string `search:"name" db:"id"`
  Owner string `search:"-"`
}

The presence of search tag in any of the struct fields should cause a document to be created with all the fields tagged with search and its id becomes the datastore key used for the given model.

Tagging db.Model with search can be used to override the index name created for the given model.

Any indexing process should happen asynchronously.