Enmet

- Changed default page caching period from forever to 30 days
- Fixes related to site changes

- Fetching images for bands, artists and albums (see documentation for details)
- Updates to fix issues related to Metal Archive site changes
- Rework of internal hashing mechanism

- Added BandStatuses enum; Band.status now returns BandStatuses object
- Enable object caching for more classes
- New properties:
- Band.links_official
- Band.links_official_merchandise
- Band.links_unofficial
- Band.links_label
- Band.links_tabulatures
- Album.last_modified
- Album.other_versions
- Artist.last_modified

- Added to Album: guest_session_musicians, other_staff, additional_notes
- Added to Artist: active_bands, past_bands, guest_session, misc_staff, links
- Extended ExternalEntity to have properties other than name (context dependent)

Fix for search_bands failing when countries parameter is used.

- Added random_band(). Used mainly for testing, but may be useful for some random exploration.
- Better handling of empty values, like "Unknown", "N/A", ["N/A"], "" etc. Now all such data is returned as None or [] (empty list).
- Change repr and str for EntityArtist subclasses to use names from specific entities, not from underlying Artist. This is to avoid fetching Artist page just to display name, repr or str.
- Minor fixes
- Split code into smaller modules