Sponsored accounts and required memo checking.

• CAP-33 Sponsored Reserves
  Provides the ability for an account to sponsor the reserves of another account, and for the sponsored account
  to revoke the sponsorship.
• SEP-29 Account Memo Requirement
  When a payment does not contain a memo, a pre-check is made to ensure the recpient accounts do not require a memo to be set.
  This behaviour can be overrideen.
• Fixes parsing errors for esoteric memo_text encodings.