file object added content_created_at and content_modified_at support.
1. added fields to the file object.
2. when uploading file, set these two fields automatically.
3. allow user to set these two fields when uploading file
Avoid in the future, default times could override user set times.
Also add content_created to uploa new version of file api.
Handle UnSupportedEncodingException
Merge pull request #19 from box/upload

file object added content_created_at and content_modified_at support.