Commit EtherCalc mode documents as SGML

[amir-zeldes]

Just like the XML editor can commit XML to Github, spreadsheet mode should commit a persistent version of the data to Github. The format should be CWB/TreeTagger SGML, so we can represent conflicting spans and convert automatically to ANNIS.

• EtherCalc format should be generated from socialcalc format in modules/ether.py using the EtherCalc GET API
• Metadata should be added to wrap the resulting document in an SGML tag with key-value pairs corresponding to all metadata fields. There is already a function print_meta() in editor.py that fetches meta data in the beginning; that part should probably be folded into modules/gitdox_sql.py and reused by both functions.

[amir-zeldes]

@Purplatypus This went into master; can you revert this commit in master and push it into dev?

[amir-zeldes]

implemented in 058cb9a