Simplify IIS hosting config

[sweetlandj]

Investigate using an IHttpHandlerFactory that will create and cache PlatibusHttpHandler instances dynamically based on the baseUri attributes specified in PlatibusConfigSections in web.config.

• Support relative paths in baseUri (e.g. "~/platibus")
• Match HttpContext.Request.Path to relative paths in config sections and construct a baseUri dynamically
• Catch and return PlatibusHandler instances based on the appropriate configuration sections

This will enable support for multiple instances of Platibus in a single IIS-hosed web app, and will eliminate the need to ensure that the host, ports, and paths for the site, PlatibusConfigurationSection, and configuration all match.