Added MODPATH to the system module

Added the MODPATH constant to the system module. (It was
previously only available as an entry in $.path_modules.)
Fonts are now installed along with the EEL modules

The PNG SFonts used with EBGUI are now installed in eelium/fonts
in the modules directory, and the tests, the default EBGUI theme,
and text.Open() (with no filename) will load "microfont_sh.png"
from there.

"microfont.png" is still included, although no tests use it.

Closes #41.