Support app_store_category config var for OSX apps.

Allows you to set app_store category configuration value instead of doing this:

app.info_plist['LSApplicationCategoryType'] = 'public.app-category.developer-tools'

This allows you to specify:

app.app_store_category = 'developer-tools'
 - or -
app.app_store_category = 'public.app-category.developer-tools'
Rename @app_store_category to just @category
Added a default value for LSApplicationCategoryType (utilities). Extracted hash to its own method.
Updated code based on Laurent's suggestion.
Fix typo - LSApplicationCategoryType
Merge pull request #115 from markrickert/add-app-store-category

Support app_store_category config var for OSX apps.