GROOVY-4572: Provide File method to get directory size

https://jira.codehaus.org/browse/GROOVY-4572

Original patch created by Evgeny Goldin. I just ported the patch to 2.0 and did some minimal refactoring. All credit belongs to Evgeny Goldin.
Merge pull request #101 from PascalSchumacher/GROOVY-4572_File_directorySize

GROOVY-4572: Provide File method to get directory size
merged