Revert "Merge pull request #344 from layerssss/master". It breaks login.

This reverts commit d1da2971144fe16a7bad1da0c79d15ad91bc4a7a, reversing
changes made to 5d14219b91c82af6de7a102db75635a27fa081a1.
lx download name:xxx (same as lx download raw:xxx)
refactor page_size
lx list --limit=NUM
refactor read_all_tasks
fetch pages on demand
automatically set limit
fix limit in OnDemandTaskList (but still buggy code)
remove login check for better performance
lx list --page-size=10
fix auto-login
fix auto login for `lx list --expired`
fix add_torrent_task_by_info_hash2, the hash must be capitalized
increase torrent file limit from 2M to 3M
add a link to https://github.com/iambus/xunlei-lixian-web
fix #371: lx delete --failed
minor change
fix #374: add verificiation code when adding a batch of tasks
lx config verification-code-path ...
add verification code when adding bt task