Added tags_string method
Tags are now persisted in a tag called "tags" rather than "tags_array", which is much more natural and inline with what other tagging gems do.
Refactored "context_array" and "array_field" to "database_field" and "field" respectively.
Updating README
Merge branch 'master' of https://github.com/lgs/mongoid_taggable_with_context into use_array_as_base

Keeping version as 1.0.0