Add MetadataMapping feature

We want to be able to map metadata to a wiki-based alignment.
This is done as a PostProcessing method.
Use Photo data structure for holding Metadata

The Pywikipedia data ingestion module uses the so-called 'Photo'
data structure. We want to use it to spare time and glue, and thus
update MetadataRecord and other methods to use it.
Use DataIngestionBot module

Overloading a few methods for our purpose
Add dry-run to UploadBot

We want to ahve a debug mode which does not upload files.
Smart postprocessing of metadata values

When postprocessing a metadata values, we do not want to erase
lists.
Add Metadata alignment

We want to be able to extract alignment base from metadata collection,
and the reverse.
Fix imports