Transition to daru-io

[zverok]

Should be done in daru-1.0 branch:

1. daru-io becomes hard dependency for daru
2. All Daru#from_something and Daru#to_something functionality should be removed from daru gem, alongside with corresponding dependencies and tests.
3. Documentation on import/export should explain clearly where to look for importing and exporting data.
4. Make sure no functionality (even minor/outdated) will be lost, at least without good reason for it. Interface changes, on the contrary, accepted and welcome.

@athityakumar, it is for you :)

[athityakumar]

1. Documented as a to-do in issue #434
2. Taken care of, by PR #430 (yet to be merged)
3. Would it be enough to include something like this in daru's YARD docs?
4. I think there has been no lose (Except the Marshal methods, as suggested in PR #430). Any suggestion for interface changes?