WIP : combine_evoked

that's a follow up on a conversation with @mshamalainen in Helsinki about contrasting evoked. Weighted contrasts should be an option and weights with nave is not always the good approach eg. in an oddball task.
And it seems like `weights` and `use_nave` are redundant, I prefer `weights='nave'` (maybe the default), `weights=None`, or `weights=[...]` for the three use cases.