copy from http://www.mozilla.org/en-US/thunderbird/features/
add section id
thunderbird button to Chinese; apply feature-links css & images
apply css for #top-features
fix js: #top-features hover
#top-features title: Change to Chinese
Trim .feature-links width
fix thunderbird: link jquery url
Fix thunderbird css for mobile screen size
#main-feature subtitle to Chinese
Merge pull request #121 from yuan817/master

已經把中間那塊的 #main-feature 加進去了