Add method to verify SEP0010 "Challenge" Transaction.

[abuiles]

Fixes #369

Task

• verify that transaction source account is equal to the server's signing key;
• verify that transaction has time bounds set, and that current time is between the minimum and maximum bounds;
• verify that transaction contains a single Manage Data operation and its source account is not null;
• verify that transaction envelope has a correct signature by server's signing key;
• verify that transaction envelope has a correct signature by the operation's source account;

[morleyzhi]

General approach looks good!

[morleyzhi]

👍