SEP-10: client attribution support

• extend sep-0010 support: add client attribution support - see #24
• extend sep-0010 support: accept multiple signers

SEP-06, SEP-09 & SEP-12 support

adds support for:

• SEP-0006 - Deposit and Withdrawal API
• SEP-0009 - Standard KYC Fields
• SEP-0012 - KYC API

update horizon 2.5.1 support

Update for horizon > 2.5.1: the representation of "..._muxed_id" field values changes to string
see also comment to issue 22

horizon related extensions

• extend asset response for horizon 2.2.0
• add new endpoint to fetch operations for claimable balances
• add new endpoint to fetch transactions for claimable balances
• add 3 missing operations responses
• add access to _muxed and _muxed_id optional fields from horizon 2.4.0

OPT-IN support for SEP0023

Add OPT-IN support for SEP-23 (Muxed Accounts M-strkeys)
look-a-like G and M addresses
see #19
see stellar/stellar-protocol#895

support protocol 17

• adds protocol 17 support
• fixes claim claimable balance bug

SEP-0010 Support

• add support for SEP-0010 Stellar Web Authentication
• minor bugfixes and improvements

add protocol 14 support

see issue: #6

make sep-0005 functions async

as requested in issue #4

fee bump & muxed accouts support for TxRep (SEP-0011)

• add fee bump support for TxRep
• add muxed accounts support for TxRep
• TxRep bugfixes and more tests