Merge pull request #91 from ewanmellor/modified-date-null-checks

Added null checks for ModifiedDate and UTCDateModified.
Merge pull request #92 from ewanmellor/escape-auth-url-params

Escape the URL parameters in BuildAuthorizeUrl.
Implement GET /<version>/copy_ref/<root>/<path>.

This adds GetCopyRef, GetCopyRefAsync, and GetCopyRefTask to
DropNetClient.

It includes a new CopyRefResponse class.
Merge branch 'copy-ref' into tipbit-master