Merge pull request #11 from jtorrente/unusedimports

Remove unused imports
Using RepoElementFields class to refer to field names in RepoElement. This way, if Api changes, the backend breaks
Merge pull request #17 from jtorrente/responsefields

Nice idea, Good job
Class with static fields for referring the subpaths of any uploaded zip (contents, descriptor, etc) and also methods for validity checking
ServerReturnMessages class with all return messages of the server in a centralized place. Also: ServerError utility exception
Using new server errors
Making use of UploadZipStructure
Unrelated to this PR - Removing a pending "entityRef" reference