Revamp stNetwork

This PR aims to improve the stNetwork in several ways:

1.  it renames it to StellarNetwork (which is in line with our internal code conventions)
2.  it renames its file to stellar-network-service.js (also our new convention)
3.  it namespaces the events it emits to "stellar-network:"
4.  it adds a helper method `StellarNetwork.request` to make it easier to make requests against the stellar network

In addition, there were a couple of lint warnings fixed