Add ability to look up and delete file from file manager

[dinosimone]

Added ability to look up and destroy file in file manager.

[alexneigher]

@dinosimone any reason why you didn't include the functionality to upload a file to the filemanager? Or am I missing that existing functionality elsewhere?

edit: Can I help you make this happen?

edit2: @dinosimone I forked the repo, pulled your branch down, and made the requisite changes to support file upload. I haven't added the tests around this new method, but all of the necessary code changes are here:
https://gist.github.com/alexneigher/6781fed73b2c931fd3702d8ee018493a
Important Updates are in:
lib/hubspot/file.rb and lib/hubspot/connection.rb

Since I couldn't find a way to open a PR against this PR to include these changes, feel free to make the changes directly on this PR (if you think they're appropriate)

[cbisnett]

@dinosimone I know it's been quite a while since you submitted this PR, but can I get you to rebase this onto master? If not, I'll take care of it and get this merged.