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: https://dl.dropboxusercontent.com/u/3915954/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
ok at least all the render methods are grouped by the name, won't be too confusing even if these methods are probably not meant to be used by users.