Merge pull request #13 from ogdch/static-file-size

Static files must be in static bucket on S3 or their size can't be got.
Log when deleting directory.
Add try-catch around shutil.rmtree
Put oai_helper.export in try/catch
Do not fail of step file cannot be deleted
If the XPath throws an error, it did not match
Prevent MemoryError when concatenating large files
Add proper error message when failing the export
Add progress callback for boto
Make percent_cb independant of class
Add re-try block for S3 upload
Add deleting the temp dir in finally block
Further improved error reporting
Include harvest_object when saving errors