Bigsitemap

* Fixed bugs in root_url generation and url_for_sitemap generation

* BigSitemap-specific options are no longer passed through to the ORM's find method

* Fixes "uninitialized constant ActionController" error
* Fixes "Unknown key(s): path" error

* Fixes broken gemspec

* API change: Pass model through as first argument to add method, e.g.sitemap.add(Posts, {:path => 'articles'})
* API change: Use Rails' polymorphic_url helper to generate URLs if Rails is being used
* API change: Only ping search engines when ping_search_engines is explicitly called
* Add support for passing options through to the model's find method, e.g. :conditions
* Allow base URL to be specified as a hash as well as a string
* Add support for changefreq and priority
* Pluralize sitemap model filenames
* GZipping may optionally be turned off

* Normalize path arguments so it no longer matters whether a leading slash is used or not