Refactoring: moved Slic3r::GCode::Base to Slic3r::GCode::Writer
Renamed shift_x and shift_y into origin
Minor cleanup and refactoring in Slic3r::GCode
Added comment
Minor cleanup of imported symbols
Some minor refactoring and cleaning to the travel_to() method and new init_external_mp() method
More refactoring to GCode.pm: make writer() public and fix usage of Slic3r::GCode without a Slic3r::Layer object