fix gravity description
add limit of vars
fix bug which object-c sdk icon hide in download page
Merge pull request #221 from forrest-mao/doc/developer

fix gravity description
Merge pull request #222 from SunLn/fix_ios_icon_not_show_bug

fix bug which object-c sdk icon hide in download page
Merge pull request #223 from longbai/new-android

android sdk
sdk document