First shot in removing _index_and_traverse_command

Remove also the blind chain of _to_index/_to_deindex_params
Locks tests are broken. To be continued...
Implement hook in test models for testing locks
First version of HashField (cf #55)

Work in progress
Rename HashableField in InstanceHashField (cf #55)
Add HashField in fields.__all__ (cf #55)
Typo cf #55
Split _reset in _reset and _del (cf #55)

"if result" behaviour can't be really controlled.
Remove distinction between full and partial modifiers
Fix linsert command

We cannot call _add, because not all the args are values
to be indexed.
Fix _del doctstring
Do not fail on non existing hdel arg (cf #55)

This is what redis does.
Split tests/model.py
More unittests for StringField
iwyu
Use assertCollection in SorteSet unittests
More unittests for indexable SortedSet
Add sanity unittests to check num calls of non indexable StringField
Use asertCollection in SetField unittests
Use assertCollection in ListField unittests
Add a unittest for ltrim command