Account for burnt CargoX tokens

[janezkranjc]

CargoX burns tokens as part of its business model and token mechanics. In this PR I added logic that subtracts the burnt tokens (sent to 0x0 address) from the circulating supply.

[valentinbora]

@janezkranjc technically, the getTokenInfo Ethplorer method should provide the circulating supply already subtracting burnt coins (so I'm concerned about a potential update there, which might determine us to subtract twice). Do you think we should differentiate between circulating and available supply here?

In any case, both on Ethplorer and Etherscan, the circulating/available supply is updated manually AFAIK.

Let me know your thoughts. Thanks!