Add _totalMinted Function

[ahbanavi]

Add _totalMinted function, as discussed in chiru-labs#124

I add tests for these situations:

• For both start token at 0 and 1
  • Before mint must return 0
  • After mint return correct value
• For ERC721ABurnable
  • Burning must not affect return value.

Also, added a comment for totalSupply() to encourage using _totalMinted() instead (for checking minted counts).

[Vectorized]

@Pczek You there? :)