add support for python 2.7

.travis.yml

beem/account.py

beem/aes.py

beem/amount.py

beem/asset.py

beem/block.py

beem/blockchain.py

beem/blockchainobject.py

beem/comment.py

beem/discussions.py

beem/exceptions.py

beem/instance.py

beem/market.py

beem/memo.py

beem/message.py

beem/notify.py

beem/price.py

beem/steem.py

beem/storage.py

beem/transactionbuilder.py