bugfix - transaction propagation to use post and remove body key from params

Bugfix changes:
1. Change transaction propagation to POST rather than GET
2. remove the key ```body``` in post params (as that makes the body to become { body: { rawTxHex: '01000...' } } )

Semantic Changes
1. Change Transaction.create to Transaction.propagate to align with API documentations and js client library. In general, I think ```.create``` gives the impression that we are making something (actually composing a bitcoin transaction) rather than propagating.

All tests passed and in the process of setting up continuous integration.