Refresh tokens support about dropbox_api

[massongit]

PR: #83

[tomekdrazek]

Hi, I added the simplest I think implementation of the refresh tokens. I checked them out against new project with short lived tokens and seems to work fine. The only downside is you must create your own implementation of DropboxAPI::Token in order to provide token serialisation routines. The README.md has details how to make a new handshake to generate offline tokens with refresh ability. Any feedback welcome.

[Jesus]

Done in #88

[massongit]

We take some time from the release of the new dropbox_api version to the the reflection on our system.
Therefore, if a similar situation occurs in the future, we want you to release the new version at least one month before the deadline.

[Jesus]

This is an open source project, if something like this occurs in the future you're welcome to contribute in advance.

[massongit]

We can create a PR. In fact, in this case, tomekdrazek created #83 on July 13th, two months before the deadline.
However, only you can merge and release the PR, so we need your help so that we can handle it on a generous schedule.