Use fields for keys/values in DictField

[lafrech]

Closes #99.

[coveralls]

Coverage decreased (-12.5%) to 83.77% when pulling 9d38ea7 on dict_field into 9485f02 on master.

[coveralls]

Coverage decreased (-0.5%) to 95.723% when pulling 4282b0b on dict_field into 9485f02 on master.

[lafrech]

This implementation allows the user to optionnaly use fields for keys and/or values.

Any type of field can be used, so it is up to the user not to pick a field that doesn't make sense for a key. Generally, fields that serialize immutable stuff serialize as string, so it should work, but then the user should be careful when querying (see #99 (comment)). Nothing really new (see #185).