For abelian fields, only compute character group if the conductor is small enough or the page may take too long/time out.
For a quadratic field, compute its Dirichlet character directly for speed.
Use conductor cutoff for computing Dirichlet characters of quadratic fields.  We can compute the characters, but the links would be useless.
Merge pull request #148 from JohnCremona/master

fixed e.c. isogeny classes for Sage version 6.2+
added Q-curves to TOC
Merge pull request #170 from davidfarmer/addQcurvestoTOC

added Q-curves to TOC
Merge pull request #169 from jwj61/master

Conductor bound for computing Dirichlet characters of an abelian field