New Gallery image and index
Added plt.draw() statement to show() for interactive use
Add support for Hershey line fonts

Hershey fonts are very simple and line only fonts under public
domain.
Added build/ folder to gitignore
Updated core.Elements to accept other Elements to add
Merge pull request #1 from mabl/hershey

Add support for Hershey line fonts
Merge branch 'master' of https://github.com/hohlraum/gdsCAD