New: DropboxUploader Error-Codes

- Each Exception thrown now has an integer error-code.
- Each error-code has a DropboxUploader class-constant.
- Each error-code bitwise-and DropboxUploader::FLAG_DROPBOX_GENERIC equals TRUE.
- See Issue #11
Version bump v1.1.11
Merge pull request #13 from hakre/development

Maintenace v1.1.11