Migrate skycoin-explorer to skycoin/skycoin repo

[0pcom]

Apparently no one has access to merge PRs in this repo ; or else the required travis CI tests are not happening - thus merging is blocked.

The source code here will be included on the cobra-cli branch of skycoin/skycoin and senyoret's work will be manually merged into that (approved by synth)

When that is done, the skycoin-explorer will be included in the command structure & made available via the default compilation of skycoin on the cobra-cli branch:

go run github.com/skycoin/skycoin@cobra-cli

it will also be included in the default compilation of skywire on develop:

go run github.com/skycoin/skywire@develop