Move interactive prompt to generate access token into client

[jhunschejones]

Description:

This resolves #12 by moving the interactive prompt to generate an access token inside of the client module. This will allow users to require the gem into a Rails console or IRB session and run Pcloud::Client.generate_access_token to generate the prompts, instead of having to download a separate script.

Testing:

This script currently does not have any unit tests in the repo, so moving it into the gem code itself felt like the right time to add them. I used WebMock when converting the code from Net::HTTP to HTTParty to make sure my web request was the same before and after the change. I decided not to leave it in the final test as that dependency feels heavy for just one use.

[jhunschejones]

I tested this in a local project, configuring an access token and then using it to access my pCloud account 👍🏻

I think since I don't expect to iterate on this logic, a simple run through here is a good enough test to confirm it works for now. I can look at adding unit tests later if a regression or bug proves we need them.

[jhunschejones]

Rebasing this on the latest version of master + adding a unit test for the new code