Separate API calls to internal functions.

[ClaytonJY]

All functions except drop_auth, drop_download, and drop_upload now use an internal API wrapper (named like api_*) to interact with the Dropbox API.

I put all the functions in the relevant file, at the end. Standardized usage with a new post_api utility function.

Fixes #109.

[karthik]

Nice! I was going to start on this over the weekend/early next week but I'll review this Monday or Tuesday. 🎈

[ClaytonJY]

@karthik cool, no rush. Some additional notes:

• no end-user apparent changes here, just refactoring.
• added tibble as an import so I could use lst; doesn't actually add a dependency since we import dplyr
• I picked api_* for the common name format of the API functions; pretty arbitrary, but they're internal, I liked keeping them separate from the drop_*'s
• post_api is kinda weird; more than open to new names. Arg order is different, but I didn't want to put the token after the ... so I didn't have to sweat naming that arg every time
• in the long-run, we should probably have strong assertions in the api_ functions; skipped for now both to minimize effort and keep this PR manageable. Save that for the new package.
• probably kills test coverage; I think most of this code was already hit in tests, but now it takes up fewer lines. PR is only net-positive on lines because of barebones function comments on new funcs.
• didn't put any real thought into doing anything with drop_auth/drop_upload/drop_download here; much trickier. Could probably share some code between upload/download, not sure about auth. Maybe post_api should have another sub-function that's even less opinionated, and could be used to build auth/upload/download requests too?