lang in Status, new rate limit control

I have committed all changes to a new branch: wenyi

I add language filed in Status class and also modify the functions related with rate limit (also GetFollowers... and so on)

so if it is ok, please merge my branch to the master one.

Changes:
(0) add lang filed in Status

(1) some code style

(2) little change in 
GetRateLimitStatus()

(3) replace 
MaximumHitFrequency() with:
GetAverageSleepTime()
GetSleepTime()

(4) add rate control in:
GetFriends()
GetFriendIDs()
GetFollowers()
GetFollowerIDs()