lx speed-test
skip files with length < 1M
refine speed-test output
remove duplicate vod nodes from list so I can priority some nodes easily
move shared code to lixian_nodes
use --node=fastest to detect fastest node automatically
make sure the default node is in the candidates
lx download --node=fast --node-detection-acceptable=1M
if no acceptable node is detected, use the fastest node
encode dirname safely
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