git tracking undo redo, tag edit/search, tests, install details

Quite a few features were added:

• git tracking now includes files and has infinite memroy git redo / undo (see README; solves issue #51)
• papers list:
  • fullsearch : can now search across all fields concatenated
  • --tag filtering, --add-tag editing and tag included in pretty papers list -1 printing (solves #30)
  • search fields can now be entered without quotes, as single words (AND or OR if --any is passed)
• git install is interactive and takes default from previous installs

Many tests were added (#48 ), though that is an endless quest (now about 80% coverage up from 50-60%).

Some behaviors break with 2.3:

• papers list -k now used for papers list --key ... instead of previously papers --key-only
• papers list --key now behaves like any other field (substring matching)
• papers list --fuzzy threshold is lowered (might be removed altogether in future versions => does not work well)
• papers install behaves differently (interactive, takes default from previous installs)

Can be installed with pip pip install -U papers-cli.