Remove execute permissions for clipper source files.
Merge pull request #1976 from sapir/clipperperms

Remove execute permissions for clipper source files.
Add message about directories as parameters for --output.
Merge pull request #1979 from sapir/output-dir-msg

Add message about directories as parameters for --output.
Add dir --output msg to README, too.
Merge pull request #1980 from sapir/output-dir-msg

Add dir --output msg to README, too.
Implement type checking for XS objects

Type handling is mainly done using templates.
Template Slic3r::ClassTraits is used to store info about exported types (perl class name). Currently only perl class name and refference name is used.
Template values are initialized by REGISTER_CLASS macro. This macro is used in .cpp file of class ( it needs to be used exactly for each type).

Ref<type> class is used to return value as perl reference. Operator overloading is used to make c++ and XSpp happy, only pointer value should be possible to return.

Clone<type> class is used to return copy of value ( using new and copy constructor). Copy is created on assigment, this should be probably improved (memory leak on multiple assignments).
It is overloaded to be able to return type, type* and type&.

Typechecking in ExtrusionEntityCollection updated to check all passed types.
Merge pull request #1983 from ledvinap/feature-xs-types-final

Implement type checking for XS objects
Update credits. Mention Gary Hodgson, Petr Ledvinap, Y. Sapir
Convert Extruder object (mostly) to C++ with lots of fields.
Make C++ gcode generator use C++ Extruder object directly.
Add XS Extruder files to manifest.
Correctly convert extruder_offset to Point object before passing to C++ code.
Replace multitude of Extruder fields with querying a PrintConfig object by id.
Add REGISTER_CLASS call for Extruder class.
Keep pointer to PrintConfig instead of copying it.
Constify ConfigOptionVector::get_at.
Replace Extruder::config accessor with Perl wrapper, with C++ option accessors.
Add xsp wrapper for Pointf class.