Initial commit
Bugfix.

show message when add duplicate word to wordlist
add vocabulary help
some enhancement to "vocabulary help"

1. position can be changed in options page
2. collapse/expand
3. avoid network delay
update version number
add keyboard shortcut to vocabulary help
use indexedDB to store vocabulary data (improve performance)
use MutationObserver to monitor popup
style change
Code Refactor(to both support chrome and firefox)
add query timeout option
Improve usability:

1. Add reload button when network problem
2. Remember expand status
add examples from vocabulary.com
hide chinese defination when vocabulary information is avaliable
Merge remote-tracking branch 'origin/master'