Merging Sage-5.12.rc0 and sage-git fixes

In addition to the newest rc0, this merges
* the "make download" target
* updated ".gitignore" for various temporary files,
* updated dev scripts
Merge important bugfix for dev scripts
Latin Squares
Todo notes for further constructions
Raise type error for number field morphisms of non-embedded number fields
trac #15521: Deprecations: default LP variables will become real instead of nonnegative
Simplify the logic for finding coercions between embedded number fields
Trac #14993: update matplotlib to version 1.3.1

This version of matplotlib has new dependencies, so create new
packages for dateutil, pyparsing, six, and tornado: all
straightforward, no patching required.
Trac #14993: update dependencies for the dateutil package.
Trac 14993: remove extraneous backquote character in matplotlib/spkg-install
improve support for PARI power series
Merge branch 'ticket/15549-improve_gen_construction' into pari_series
Trac 13774: Irreducible Cartan types created as reducible
Review patch for #13774.
trac #15268: Rebase on 6.1.beta2
trac #15286: Reviewer's comments
trac #15286: Reviewer's comments 2
check for exact 0 in series substitution
trac #15627: (disjoint) Union of graphs with multiple edges
trac #15333 remove import of is_* in modules/all.py