Create dom.Error interface and dom.SEVERITY_* constants
Create dom.ErrorHandler interface
Create dom.Locator interface
Merge remote-tracking branch 'origin/error' into locator

Solves issue #8: Create dom.Error interface
Conflicts: 1-4-Fundamental-Interfaces.go
Merge pull request #32 from go-turbo/error-handler

Solve issue #9: Create dom.ErrorHandler interface
Fix indentation
Merge remote-tracking branch 'origin/locator' into node

Solves issue #14: Create dom.Locator interface
Conflicts: 1-4-Fundamental-Interfaces.go