Retry yahoo DataReader on 404 up to 3 times
This should fix bug #826 that had yahoo randomly failing downloads.

Hey Paddy, thanks for the pull request! It would be nice to have two additional keywords, "retry_count" and "pause" to give further control for retrying. You can set the retry_count to 3 by default and pause to be 0.
 I added retry_count and pause through DataReader.  Looking at the code it doesn't look like this will break any of the other ways that DataReader is called.  Are there testcases for the other uses?
 I don't think so. I actually plan to deprecate the `DataReader` class in favor of independent functions for each data source
 Merged / pushed this. A reminder that you should make changes in feature branches and not merge the upstream branch. You'll probably want to fetch upstream and do:

`git reset --hard upstream/master`
