Phconvert

What's new?
- Fix always-True assertion when loading SPC files

What's new?
- Documentation improvement and new API docs: http://phconvert.readthedocs.org/
- Add notebook to write Photon-HDF5 files from scratch: [Writing Photon-HDF5 files](https://github.com/Photon-HDF5/phconvert/blob/master/notebooks/Writing%20Photon-HDF5%20files.ipynb)
- [`save_photon_hdf5()`](http://phconvert.readthedocs.org/en/latest/hdf5.html#phconvert.hdf5.save_photon_hdf5) improvements:
- Restore input dict sanitization (it was mistakenly disabled)
- Cast bools to ints for improved compatibility
- Auto-compute acquisition_duration for single-spot data
- When not provided, create and populate `identity` group
- Improve Photon-HDF5 validity test (852242c05864f8cc5a064b00b1c7efac26dfb581)

What's new?
- Fix a bug causing file validation to fail (fd0a278679e6d33ee15c9acc5ff194c03e6f36b0)

What's new?
- **Rewrite of validation function**: now they validates a HDF5 file, not an in-memory representation. This makes easier to test if a file is Photon-HDF5 compliant, regardless if it has been created by phconvert or not.
- Added more tests during validation
- Fixed error that caused authors and other identity fields not being saved

What's new?

A few important changes:
- Automatically convert input of "scalar" fields to scalar if they are arrays or sequences of length == 1. See 5.
- Check conformance with the specs in `save_photon_hdf5()` removing the need to call `assert_valid_photon_hdf5()` separately.
- Remove `time_reversed` field in `nanotimes_specs`. Now we use the simpler convention that `nanotimes` has a time axis direction correctly oriented. See 6.

What's new?
- Updated the descriptions (`TITLE` attributes) for various fields.