Add support for WPMU and fix ABSPATH autodetection.

Added support for two new cmd params:

  --host
    Indicates the hostname to use. Required to properly deal with WPMU installations. For example: --host=blog2.example.com

  --uri
    Indicates the URI path to use. Required to properly deal with WPMU subfolder installations. For example: --host=blognetwork.example.com --uri=/blog2

Also changed the way ABSPATH is determined to work in Win systems properly, and first try the location used by a default plugin install.