Remove unnecessary directory structure

Remove rails4 directory
Remove shared directory

Consolidate railtie.rb files
Remove unnecessary defined? wrapper around Railtie
Remove comments [ci skip]
Format hash
Remove rdoc from gemspec
Move PostGIS 1.5 notes to separate file
[ci skip]
Version 2.0.0
Update history [ci skip]

Add 2.0.0 release notes
Add 0.6.6 release notes