Features/add list all files method

[bunnymatic]

Add a list_all_files method to Client

• uses the files/list_folder/continue endpoint with a cursor
• replaces /delta from v1 - https://www.dropbox.com/developers/reference/migration-guide

[bunnymatic]

I wasn't sure how you feel about this method _list_folder... It seems like the call to request('/files/list_folder') could be pulled out as a little helper since it's now used twice, but maybe not worth the overhead.
So in this second commit, i took that little refactor out.

[bunnymatic]

I noticed that #1 takes the first step towards this end. I also looked at solving this without modifying the interface, but because the methods return an array of items, the parameter that I need for this recursive refetch (has_more) is lost when the response entries are extracted and wrapped in the Dropbox::Metadata class.

https://github.com/waits/dropbox-sdk-ruby/blob/master/lib/dropbox/client.rb#L131

Have you considered making methods return the rest of the response in someway?

[bunnymatic]

Kind of looks like I should move to another gem as this stuff hasn't been touched in 7 years. As I am using this branch, it'll likely stick around for a bit on the fork at rcode5, but I'll close this PR cuz it's clearly not going to get brought in.