Implement font method

Addresses Issue #273.

Implements the font method in builtin_methods.rb.
The font method in builtin_methods.rb news up an instance of the Font class.
The Font class first checks to see if the requested font is available on the current platform.
The Font class then checks to see if the requested font is already in the fonts folder.
The Font class loads the requested font from file system to the font folder for use if it is not already in the font folder.
The Font class also populates the Shoes::FONTS constant with an array of font names currently available.

There seemed to be some shared functionality needed for the Font class and the FONTS constant so I worked on implementing them for both.

This is an early pull request and doesn't close issue #273.
Still have more work and refactoring to do.