fix upload bt
增加两步输入验证码的方法

* 之前的 --verification-code-path 机制需要在命令行中马上输入验证码，给它做web界面时发现很难集成
* 现在加入两步输入验证码的方法，如果加入--verification-code-input-later参数的话，第一步就仅 check
并获取图片，第二步再输入验证码。
* 原有的 --verification-code-path 的行为没有改变，还是在命令行中询问用户输入验证码。
验证码这个问题已经解决了，把它加入文档
Merge pull request #344 from layerssss/master

增加两步输入验证码的方法
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