Fix caching and concurrent operations

• Fix regression introduced in 1.4.3 appearing when
  synchronising big repositories (#38)
• Fix concurrent read/write by using an atomic rename (#35)
• Tree objects can also point to commits (@codinuum)
• Reduce allocation (@codinuum)
• Use LRU cache instead of an unbounde Hashtl
  (code imported for Simon Cruanes's CCache implementation)
• Remove the crazy unbounded caching in Git.FS. Use the LRU
  everywhere (#22)
• Fix fd leaking (#29)
• Update to dolog.1.0
• Remove dependency to camlp4
• Remove lots of warnings
• Move Git_unix and Git_mirage in their own subdirs as it
  was causing issues to oasis (#5, Simon Cruanes)
• Use Bytes instead of String (#5, Simon Cruanes)