add dataCenter option when assign file keys
add dataCenter option when starting volume servers
some work related to freeze a volume. Not tested yet.
add error when size limit is reached
Issue 26:	Could not download uploaded files

Fix is provided by hieu.hcmus, He convinced me of the fix. Thanks!
change default number of volumes to 7, so that the first volume growth
would not fail.
Issue 27:	feature request - Last-Modified header
bump up version number to 0.32
1. ensure replicated file has the same timestamp
2. upload can specify modified time by &ts=...
3. correctly return code 304
remove unused code
Issue 28:	[HELP NEEDED] weed upload does not send correct mime type to
weedfs

Fix is provided by claudiu.raveica
ensure mime type checking by lower cased file name extension
avoid writing the same file
"weed upload" return message changes
empty deleted file
write file info only when data is not empty
merge cdb
slight premature optimization for volume choosing
re-merge origin, forget most changes