Additional metadata fields

[zgornel]

This PR:

• Adds additional metadata fields to documents: id (document identifier), publisher, edition_year, published_year, documenttype and associated functions for mutating, obtaining the fields and tests.
• Bugfixes: Imports Base.show, Base.names and removes outer 'using DataFrames'

The new fields might prove useful if working with large sets of documents (i.e. publications, books) where these fields can provide helpful in creating subsets of documents based on their values. The changes are innocuous by themselves as backward compatibility with existing code should be ensured.

PS. changes from commit 7f5fc87 were discarded when rebasing on the latest master

[zgornel]

Closing this pull for lack of feedback.