Implemented basic importer function.
Fixed name of the file with data from 'user' table.
Fixed default value of show_gravatar attribute.
Combined similar operations in import_data function.
Checking if there's any data in tables before importing.
Fixed importing issues caused by incorrect order of columns in exported archives.
Small fixes for previous commit.
Fixed order of columns for 'user' table importing.
Added docstrings for exporter and importer commands.
Added a note about possible archive importing improvements.
Merge branch 'importing'