Add modify age and editor to signatures

When there are multiple pilots scouting a single
system in parallel, it helps to see who modified
a signature and when. Editing a signature can be
thought of as claiming for ownership so that
collisions in scouting work are minimized.

Signed-off-by: Mika Kuoppala <miku@iki.fi>
Change cache handler to cache XML rather than eveapi object.
Merge branch 'api_cache_change' into develop
Merge branch 'sigmodage' of https://github.com/mkuoppal/eve-wspace into sigmodage
Use naturaltime instead of model method. Add migration. Move time to mouseover.
Merge branch 'sigmodage' into develop