Added null checks for ModifiedDate and UTCDateModified.
Remove RestSharp DLLs from Lib and packages directories to make sure that
we don't build against them.  We're using our own build of RestSharp.

Update DropNet.csproj and DropNet.Tests.csproj to refer to our local DLL.
Escape the URL parameters in BuildAuthorizeUrl.

In particular this means that the callback URL passed in to this function
and DropNetClient.GetTokenAndBuildUrl will be correctly escaped.
Merge pull request #1 from ewanmellor/escape-auth-url-params

Escape the URL parameters in BuildAuthorizeUrl.
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
Implement POST /<version>/fileops/copy using from_copy_ref.

This adds CopyFromCopyRef, CopyFromCopyRefAsync, and CopyFromCopyRefTask
to DropNetClient.
Merge branch 'copy-from-copy-ref' into tipbit-master