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