issue #3: fixed bug that analyzing an empty Groovy file raised an error

- the error was raised because CodeNarc's StringSourceAnalyzer
  asserts that the string to be analyzed is not empty
- now also accessing a file's content directly instead of
  creating a compilation unit first
issue #3: Grails plugin sources are now excluded from code analysis

- using the linked resource '.link_to_grails_plugins'
  (automatically created for a Grails project in Eclispe)