Add callback to createPackage

This allows us to create packages programatically.
fix minor typo

insertDirectoy to insertDirectory
Add callback to disk.writeFilesystem
fix only call callback when function
minor fix missing var statements
Merge pull request #7 from bwin/fix-typo

fix minor typo
Merge pull request #5 from bwin/add-callback

Add optional callback to asar.createPackage & disk.writeFilesystem
Merge pull request #8 from bwin/minorfix-missing-var-statements

minor fix missing var statements
0.1.2