change default mapnik fontdir_recurse to true

[amandasaurus]

Changes the default value of fontdir_recurse from false to true. (i.e. by default tirex-backend-mapnik will recurse through the font dir, unless the user turns that off). IME, this (recurse=true) is the common configuration, so I think that should be the default.

I am not 100% sure if this patch is correct, and ask for a review.