Add datapackage class, update create_package() and check_package()

[peterdesmet]

Fix #184, #185

• Add datapackage class to package and describe it throughout as a "Data Package object"
• Allow to provide an existing descriptor to create_package() to make it a Data Package object. Use it in read_package()
• Always run a check_package() at the end of create_package()
• Make check_package() public and provide more meaningful error messages. Warn if datapackage class is missing, so that packages using frictionless as a dependency don't break.
• Don't set "profile"="tabular-data-package" and remove it from example dataset #188

[PietrH]

Tiny suggestions, nothing that needs to be changed per se.

Very nicely done, very clear and easy to review.

• I've found another #156 and fixed it. 4f228ac
• Currently in the differences with datapackage.r section of the readme we mention the fact that datapackage.r outputs a Package object as a major difference. This is no longer true.

[peterdesmet]

@PietrH implemented most of the suggested changes. Can you review again?