ENH: draft of mne report command

Closes https://github.com/mne-tools/mne-python/issues/1056 

Try
```sh
 mne report -p MNE-sample-data/ -i MNE-sample-data/MEG/sample/sample_audvis-ave.fif -d MNE-sample-data/subjects/ -s sample
```
and it should generate ``report.html`` in ``MNE-sample-data/``

Sample output here: http://neuro.hut.fi/~mainak/report.html

TODOS
----------
- [x] recursive exploration
- [x] rebase and use read_evokeds
- [ ]  extend the support to as many fif files types 
       - [x] cov
       - [ ] fwd 
       - [ ] inv
       - [x] raw
       - [x] trans (display head in helmet to check coregistration quality in ``mne.viz.plot_trans()``)
- [ ] check dpi settings
- [x] Slicer coordinates
- [x] Table of contents linking to different parts of html?
- [x] the bootstrap/JS theme should allow to select which type of fif file to display. See jquery toggle
- [x] also bad fif files should appear in red for example if the fif fname is not standard (evoked should end with -ave.fif, cov with -cov.fif, raw with raw.fif or sss.fiff etc....)
- [x] banner + footer
- [x] open the report in a browser when generated
would be cool to add a `render_custom` function here that takes arbitrary callables / user defined stuff