Bigsitemap

* Separate URL and file paths are now supported via the :document_path
and :url_path options
* Fixes an issue when initializing in Rails 3

* Fixes an issue where sitemap files were not being generated if the same model
was added more than once (fixes issue 5: https://github.com/alexrabarts/big_sitemap/issues/issue/5)

* API change: Rails/Merb are no longer automatically detected - use BigSitemapRails and BigSitemapMerb instead
* API change: Rails' polymorphic_url helper is no longer used to generate URLs (use a lambda with the new :location option instead)
* Static resources can now be added using the add_static method
* Incremental updates are now available via the :partial_update option
* "loc" URL values can now be generated with lambdas
* Sitemap files can now be locked while being generated using the with_lock method
* Several bug fixes

* Fixes an issue with the :last_modified key being passed into the find method options

* Add support for lambdas when specifying lastmod

* Use Bing instead of Live/MSN. Note, this breaks backwards compatibility as
the old :ping_msn option is now :ping_bing.