Update Fourier expansion framework to Sage 4.8.

1) Update all documentation and Latex string in the Fourier expansion framework.
2) Integrate Siegel and paramodular forms.
3) Split the construction functors for monoidpowerseries_ring and _module.
   This change is neseccary since the category framework in Sage 4.8 enforces the codomains of functors.
Merge pull request #2 from martinra/modforms_squash

Update Fourier expansion framework to Sage 4.8.