Refactored InodeFolder to store children Inodes rather than ids

This simplifies many operations
Commented InodeFolder and cleaned up MasterInfo
Fixed docs and naming in InodeFolder
Small fixes to InodeFolder
Improved comments and a parameter name
Removed unnecessary addToFileIdPinList method
Merge pull request #178 from manugoyal/inode-folder-change

Refactored InodeFolder to store children Inodes rather than ids
fixes
Merge branch 'master' of github.com:amplab/tachyon