In-person code review

[fulldecent]

This issue tracks Will's notes from our code review.

@MoMannn please add the notes here you have so I can track them.

If any of that stuff is a security concern for you and you don't want to publish it right now then still post it but just encrypt it (https://superuser.com/a/370412/38689) and send me the key separately.

[MoMannn]

Notes (with my comments):

• check other valid token implementation
  (I have not found a different implementation - the only difference I found was either in the name or that it was not a modifier but an function)
• owner override problem (check remix)
  fixed
• check if validNFToken is necessary in functions (to lower gas consumption)
  I determined that it was necessary
• assembly check in a seperate function/library
  fixed
• require in clear approval (is it necessary)
  fixed
• mint return true - not necessary -> remove
  fixed
• document proof -> make it private/check other implementations
  it needs to be internal for burnableXcert, added notes that is has to exist for every token.
• remove string check
  fixed
• TokenURi check
  fixed
• eip165 into seperate contract?
  fixed
• function order
  it is order by functionality
• make style guide public
  @xpepermint can you do this?

/pull/24

[fulldecent]

All issues addressed, closing