StaticFileHandler improved Range support.

StaticFileHandler.cs:
-"Accept-Ranges: bytes" was not set which resulted in IE not being able
to seek videos.
-If user seek video Chrome sends header "Range: bytes=5000-" which
resulted in sending whole file because "rangeEnd != fi.Length - 1" was
true but rangeStart was not 0.
-Ignoring exception "An operation was attempted on a nonexistent network
connection" because it just means browser disconnected
MimeTypes.cs:
-Added most common video types(mp4, ogg and webm)
EndpointHostConfig.cs:
-Added mp4, ogg and webm to AllowFileExtensions as default