BUG: special: fix a number of bugs in Mathieu functions

This PR fixes a number of issues in Mathieu functions:

- Out-of-bounds write (and SEGV) for large-order functions

- Return correct values also for `q < 0` (specfun implements only q >= 0; use symmetry relations)

- Some corner-case value fixes

- Add tests against values obtained from GSL. (GSL is GPL-licensed, but I don't think this puts any restrictions on using values generated by it. The value dumper program, however, has to be GPLed...)

Closes Trac [#1465](http://projects.scipy.org/scipy/ticket/1465), [#1468](http://projects.scipy.org/scipy/ticket/1468)