Add GetSize method to Filename class.

It returns size of the file, if it exists in file system.

Filename class currently uses a lot of `stat()` function without caching
it's results. We need to implement some kind of cache for it.
Use Filename class in Include method.
Add support for directory index files in HTTPD.

Both `index.tly` and `index.html` are now supported.
Merge pull request #32 from peelonet/httpd-index-files

Add support for directory index files in HTTPD.