Use NSWorkspace instead of SBApplication in Open terminal.sketchplugin

- Use NSWorkspace instead of SBApplication in Open terminal.sketchplugin(fix almonk/SketchGit#6)
Move 'getCurrentDirectory' function, and add 'executeCommand' function

- Move 'getCurrrentDirectory' fucntion from 'com.bomberstudios' to
'net.khasegawa'
- Add 'executeCommand' function on 'net.khasegawa'
Use net.khasegawa.executeCommand on 'git init' command
Change "executeCommand" function name, rename to "exec"
Use "exec" function instead of "doScript_in_" fucntion.

Use "exec" function in "net.khasegawa" object instead of
  "doScript_in_" function member of SBApplication instance.

Changed:
- Git/Branch.sketchplugin
- Git/Checkout.sketchplugin
- Git/Init.sketchplugin
Fix 'undefined' is not a function

Fix 'undefined' is not a function.
Use "net.khasegawa.getCurrentDirectory" instead of
  "com.bomberstudios.getCurrentDirectory" function.
Use "getCurrentDirectory" function instead of "getFildeFolder"

Use "net.khasegawa.getCurrentDirectory" funcion instead of
  "com.bomberstudios.getFileFolder" function.
And refactor execute script commands.