[TT-6928] Support browser and node env and custom transports

Changelog

Added CircleCI pipeline for building and publishing to NPM and tagging repo
Refactored the architecture for the library, introduced new concepts Service, Client, Transport etc.
Updated README
Published (privately) an example of this new package @what3words/api@3.3.7-rc-1
Can test with a lambda using attached lambda-test.zip
Rewrote tests with Mocha and ShouldJS and added coverage with Istanbul
Addresses and fixes issues https://github.com/what3words/w3w-node-wrapper/issues/30 and https://github.com/what3words/w3w-node-wrapper/issues/29