Add python 2.6 support

[mflaxman]

This doesn't work on 2.6:
https://github.com/blockcypher/bwallet/blob/3f54eb816c5ba1a79abe64a7a914ea093784102c/bwallet/cl_utils.py#L29-L30

Not 100% sure if this will be compatible with all dependencies anyway.

Don't forget when updating to also get this:
https://github.com/blockcypher/bwallet/blob/e6a46397088f32f1632fd6d0643fce57ebb82aff/bwallet/bwallet.py#L1105