Update MimeTypes.cs

Adding Set/Remove/Clear methods for the mime-types. I've set them to protected internal for now just to be safe.

I've also added ToLower() calls to avoid any case-sensitive issues with mime-type look-ups.

These methods could be used to allow web applications to control the file/mime-type mappings via the web.config files. I may have to dig into XSP, but this is a start.

Ideally I would like to be able to support the following web.config example:
<configuration>
   <system.webServer>
      <staticContent>
         <mimeMap fileExtension=".html" mimeType="text/html" />
         <mimeMap fileExtension=".txt" mimeType="text/plain" />
      </staticContent>
   </system.webServer>
</configuration>