ERC721 contracts no longer initialize their parents.

[nventuro]

This fixes the issue with diamonds in the ERC721 inheritance tree (namely, having to pass name and symbol multiple times to MetadataMintable). Initializer for all parents have to be manually called (the mocks are an example of this).

Since we'll be providing a metadata, enumerable, mintable and pausable standalone ERC721 as part of our EVM package, this shouldn't be too bad, thought I'm not thrilled that initializers forced us to go this way.

I've also added some initialization safety checks in the style of #27.

[coveralls]

Pull Request Test Coverage Report for Build 76

• 8 of 9 (88.89%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 98.91%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
contracts/token/ERC721/ERC721Enumerable.sol	1	2	50.0%

Totals
Change from base Build 75:	-0.8%
Covered Lines:	565
Relevant Lines:	566

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💛 - Coveralls