rename and refactor (#26)

* Adds ExFSWatch.Backend behaviour

* Adds a subscription API to ExFSWatch.Worker

The Module based system is still available, it just subscribes to the worker in
a spawned process.

* Adds Subscription API to readme

* rename to file_system

* refactor the code struct and fs_mac backend

* more log for fs_mac compiler

* add module based API back

* Moves the compile step into `mix compile`

* refactor fs_linux backend

* fix build for fs_linux backend

* support delete and attrib events for fs_linux backend

* remove kqueue backend, make inotify work for both linux and freebsd

* refactor fs_windows backend

* bugfix: whitespace in file path for fs_inotify backend

* fix compile order and move backend to another module

* add whitespace in path test case

* add whitespace test case for fs_mac backend

* add test for module api and subscribe api