Fix typos
Merge pull request #7 from kavinyao/fix-typos

Fix typos
Remove cliean lease code in master.go
Merge branch 'master' of https://github.com/wweiw/simplegfs
Add lease management in chunk manager

 - Use FindLeaseHolder() to find primary's location.
 - Use addLease to add a new lease.
 - Use checkLease to see if a lease is still valid.
Add ExtendLease function to chunk manager