Client URL options and writer type.

### Accept explicit URL options

When you have Solr running on a non-standard port, you only need to specify the port instead of a complete URL. This is fully backwards compatible.

### Accept `default_wt' as an option

When you want multiple client instances with different default writer types, you can specify them during initialization instead of changing the global default, affecting all (new and existing) instances. Technically, this is not completely backwards compatible as the writer type of already existing instances could be intentionally changed by calling `RSolr::Client.default_wt=`. This will no longer have any effect on existing instances; their default writer type can be changed individually by calling `RSolr::Client#default_wt=`.