Davidslv/add stream for mixpanel response

[Davidslv]

Hey @keolo, I've modified the Mixpanel::URI.get to handle stream instead of parsing and read the whole response, which can be quite big on an export, since it's per day, a project with many events/properties or events with many properties can take quite a while to "download" everything into memory. This should close #44

Thank you

[keolo]

This seems like a great idea. This could use some specs and refactoring (e.g. try running rubocop) before merging.

[Davidslv]

Hey @keolo, I did it in a way that it didn't change the api, and still made your specs pass.
Although because of the latest, I think the implementation is not correct.
Meaning that now it downloads the file in chunks but then it still has to put it in a string variable, which will most likely cause memory issues. So I left your method alone and I created another one called download which accepts a directory and a filename so that all the writing is done in chunks to a specified file. You can check it out here: https://github.com/Superfling/mixpanel_client/blob/master/lib/mixpanel/uri.rb#L21 although we decided to not download the whole export for what we need, hence the file_size limit here ( https://github.com/Superfling/mixpanel_client/blob/master/lib/mixpanel/uri.rb#L30 ) but that can be changed easily if you decide to adopt it.

Thank you