Enhancement: Added compression to SimpleCrest cURL fallback

If zlib or pecl is installed, the Accept-Encoding HTTP header is set to
"gzip" for cURL fallback method (file_get_contents).
This fixes the problem with zKB fetching without having cURL available
(zKB API now requires to either use gzip or deflate as compression).
Feature: CREST item value fetcher

Replaced the old item value fetcher (depending on an external source for
item prices in XML format) with a fetcher for CREST average market prices
(using endpoint https://public-crest.eveonline.com/market/prices/).
This endpoint is updated daily and contains the same prices used ingame to
display item values.