Meredith Graph constructor

Conflicts:
	src/sage/graphs/graph_generators.py
Horton Graph constructor
Kittell Graph constructor
Markström Graph constructor
Sousselier Graph constructor
Poussin Graph constructor
Tutte Graph constructor
Wiener-Araya Graph constructor
Two broken links in the documentation of graph generators
Rebase on 5.13.beta0
Declare various sig_ macros as nogil
Revert parts of 67a76032fda3ee140c3bc52422020ef722cbee52
Update signals section of developer manual
Document sig_error()
Updated Sage version to 6.1.beta0
Update installation guide for installation with OS X Mavericks
Trac #15319: Update instructions for installing with OS X Mavericks

On OS X, to install Sage you have to first install the "command-line
tools". With OS X Mavericks, the method for doing this has changed, so
we need to update our documentation.

URL: http://trac.sagemath.org/15319
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Michael Welsh
Trac #15352: Declare various sig_ macros as nogil

One consequence is to allow Cython's OpenMP facilities, see #12690.

While we're at it, do lots of clean-up of signal/error handling after
#9640 and friends:
 - use `sig_error()` for NTL and libGAP
 - remove the old `_sig_on` macros which are deprecated since 3 years

Follow-up at #15543: update documentation

URL: http://trac.sagemath.org/15352
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Peter Bruin
Trac #15543: Update signals section of developer manual

This patch:
- reverts part of 67a76032fda3ee140c3bc52422020ef722cbee52
- applies the changes to `coding_in_cython.rst` which was part of #15352
with ''positive_review''
- additionally documents `sig_error`

URL: http://trac.sagemath.org/15543
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun
Trac #15044: Meredith Graph constructor

http://en.wikipedia.org/wiki/Meredith_graph

Here it is. As my first Git patch `:-P`

Nathann

URL: http://trac.sagemath.org/15044
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): Rob Beezer, Volker Braun