added support for /delta API call (initial work)

This change adds initial low-level support for the /delta API call
provided by Dropbox. It does NOT (yet) provide any convenient means
for actually maintaining a view of the remote state; you still
currently need to do this yourself.

These changes are as of yet untested.
add support for nested arrays
fixed clientModified field (API change on Dropbox side)

clientModified is really client_mtime
rm extra debug
add basic delta test
refactor private members in QDropboxDeltaResponse
updated .pri files
Merge pull request #38 from bzy-xyz/master

added basic /delta support, other fixes