Add proxy support to Dropbox Client

[mdreier]

Hi,

I am using DropPHP in a project. Since I am behind a proxy I ran into issues using it to access Dropbox, so I implemented proxy support into the library. This may be useful for others, so I am offering this change to be pulled into your project.

I'll be happy to do any further required changes if something is missing (documentation, code style, etc.).

Best regards, Martin

[micha137]

Maybe it is also a good idea to offer a generic way to set CURL options.