Enhancement: new API call: is_valid_account

[cc32d9]

get_account API call is gathering a lot of information, but in many cases the requestor just needs to check if an account exists on the blockchain.

A new API call is needed that is just returning true or false for a given account name. This will save a lot of CPU time on API servers and clients.

The feature is non-intrusive, and can be added to the current 2.0 software.

An even better option is to take an array of accounts and return an array of valid accounts. But then it needs to take care of response time limits and error handling.