slpjs should be configurable for other rest servers

[christroutner]

When slpjs is instantiated inside slp-sdk, it is not passed any configuration data. This means it calls rest.bitcoin.com and is not configurable for other rest servers. The scope of this issue is to pass the restURL value through slp-sdk and into its instance of slpjs, so that the entire library can be configured for other rest servers.

[maikelmclauflin]

seems like this issue is fixed as of: 15f854e