Implement retry policy for some API calls

Some dropbox API calls may be rate-limited in certain conditions by responding with HTTP '429 Too Many Requests' (with header 'Retry-After: 1')
May be we can set up some **'retry policy'**, to automatically handle such situation, for example:
`const dbx = new Dropbox({ accessToken: token, numberOfRetries: 10 }); `

default will be 0.