Resolve "Implement Quadkey functions"

Closes #22 (closed)

• Added all functions related to Quadkeys from https://learn.microsoft.com/en-us/bingmaps/articles/bing-maps-tile-system
• Added package quadkeyfunctions, with one function get_quadkey_functions, that reads all SQL files in the directory
• Added function implement_quadkey_functions in the PostGIS Database

\approve @ds @gislars