[ui:cmd:download] suppport mobile browser & fix for Chrome
[driver:dropbox] support curl PUT method & fix multiple upload (folder)

Note: Need extended dropbox-php liblary for curl PUT support. You can
find it from https://github.com/nao-pon/dropbox-php/tree/put_support .

And, You must be set `define('ELFINDER_DROPBOX_USE_CURL_PUT', true);`
into connector.php .

(The PullRequest is https://github.com/Dropbox-PHP/dropbox-php/pull/21)
Merge branch '2.1' into 2.1_n