Add hack sign

Hack the download sign so could download user's netdisk file
Fix pep8
Add get private file dlink
Use rstrip instead of split str array
Migrate urllib2 to requests

* login
Export migrate to requests

* login
* export
Refactor Pan class

Trying to separate operate and data(shareid, fsid, blabala...)
Move get_js method to new Pan class
Change download useragent
Fix save login cookies
Little modify
Update pattern of finding javascript
Refactor handle captcha
Refactor FileInfo
Refactor get download link
Remove album support
Add download cookies support

If without cookies, download speed will be limited.
Refactor download command
Update README