Protocol 11 Compatibility

[theaeolianmachine]

Protocol V11 of Stellar adds a new manage_buy_offer operation. Also names of manage_offer and create_passive_offer have been changed to manage_sell_offer and create_passive_sell_offer.

References:

• Protocol v11 XDR changes stellar/go#1164
• https://github.com/stellar/stellar-protocol/blob/master/core/cap-0006.md
• XDR files diff - this shows all changes between v10.3.0 and v11.0.0 in Stellar Core — you'll need to just look at the changes in src/xdr (realistically any file that ends with a .x extension).

Let me know if you have any questions!

[bnogalm]

Implemented and merged:
#27