Replace Artist.browser by Artist.create_browser and add RDOC

* lib/photograph/artist.rb:
Add missing doc about `browser` option.

* lib/photograph/artist.rb:
Update authors

* README.md:
Don't create browser if one already exists

* lib/photograph/artist.rb:
Provide a #before method to supply a hook before shooting

* lib/photograph/artist.rb:
Bump version

* lib/photograph/version.rb:
Update README

* README.md: