Add INCAR File Output

This commit gives VaspCat the ability to output INCAR files, even when
no crystallographic files are present as input.  This initial commit
gives VaspCat support for 10 INCAR variables: system, npar, encut,
ediff, ediffg, gga, ispin, lwave, and lcharg.  By default, only system
is added to INCAR output files.

To accommodate these changes, the module responsible for reading the
user config file (settings.py) was modified.  Now, if the user sets an
INCAR option equal to none, that section will not be found in the
resulting INCAR file.

CHANGELOG:
- Added vaspcat/core/incar.py module to save INCAR files.
- Modified vaspcat/backup.ini and vaspcat/default.ini to include new
  INCAR variable names.
- Modified vaspcat/output.py to call vaspcat/core/incar.py main method.

vaspcat/poscar.py: - Added atomic title output option (title_mode = 3)

vaspcat/settings.py: - Changed integer option validation from comparing
                       against a list of integers to seeing if the
                       provided integer is within a given range.
                     - Allowed the user to enter 'none' as a value for
                       options in the INCAR section.

vaspcat/extend/configuration.py: - Added 10 new INCAR variables names to
                                   check for proper syntax.
                                 - Modified integer options to have a
                                   range of numbers as possible values
                                   rather then listing every possible
                                   option and checking against the list.

TODO:
- Add KPOINTS output functionality.
- Fix pdb reading bug in vaspcat/extend/filetype.py where atom names are
  improperly read due to extra letters added as labels.
- Transition docstrings to Sphinx rst format.
- Document and add boilerplate text to modules missing these docstrings.