Use official GW2 API for the GemExchangeSpider

Since this was the spider to use gw2 sessions, they are now removed as well. There have been some other cleanups as well.

List of changes:
- remove item-db worker from start-workers.sh
- remove some now unused urls from the default config
- do not route /admin/session POST anymore. We don't use them anymore anyways
- don't try to log into a gw2 session anymore in worker-gem
- remove some unused use statements
- remove session related functions from the base spider and add a central function to retrieve API data
- use the official gw2 API to retrieve gem exchange data
- remove gw2 session related classes and the old TradingPostSpider
- change the admin_session template to inform the caller that it's not used anymore.
Update README.md

This version includes documentation changes for the updats gem-worker, an added section to install grunt and some minor changes (some hard linewarps among others).
Update BaseSpider.php

Make getApiData Exception text less specific.
Merge pull request #158 from keneanung/GetGemxchangeFromApiv2

Get the Gem Exchange dta from the official GW2 API