Refactor setup_module methods [pytest] 

Rather than adding repetitive `setup_module` methods to the sympy code base, we can make use of a single module level fixture to skip tests marked with `disabled = True` as is done for `bin/test`.  The fixture also checks for `ipython` as those tests can only be run with the most recent version of pytest or with the -s flag.

```bash
$ py.test sympy/external/tests/ sympy/interactive/tests/ sympy/plotting/ sympy/printing/
============================= test session starts ==============================
platform linux2 -- Python 2.7.8 -- py-1.4.23 -- pytest-2.6.1
architecture: 64-bit
cache:        yes
ground types: python 

collected 531 items 

sympy/external/tests/test_autowrap.py sssss.sssss
sympy/external/tests/test_codegen.py .s.s....
sympy/external/tests/test_importtools.py ...
sympy/external/tests/test_numpy.py sssssssssssssssssssssss
sympy/external/tests/test_sage.py ssssssssssssss
sympy/external/tests/test_scipy.py s
sympy/interactive/tests/test_interactive.py .
sympy/interactive/tests/test_ipython.py ss
sympy/interactive/tests/test_ipythonprinting.py sss
sympy/plotting/intervalmath/tests/test_interval_functions.py ssssssssssssssssssssssss
sympy/plotting/intervalmath/tests/test_intervalmath.py .......
sympy/plotting/pygletplot/tests/test_plotting.py sssssssssss
sympy/plotting/tests/test_plot.py .s.
sympy/plotting/tests/test_plot_implicit.py s
sympy/printing/pretty/tests/test_pretty.py .................................................................
sympy/printing/tests/test_ccode.py .........................
sympy/printing/tests/test_codeprinter.py ...
sympy/printing/tests/test_conventions.py ..
sympy/printing/tests/test_dot.py .........
sympy/printing/tests/test_fcode.py ................................
sympy/printing/tests/test_gtk.py x.
sympy/printing/tests/test_jscode.py ......................
sympy/printing/tests/test_lambdarepr.py ....
sympy/printing/tests/test_latex.py .....x.........................................................................x.....
sympy/printing/tests/test_mathematica.py .........
sympy/printing/tests/test_mathml.py .................
sympy/printing/tests/test_precedence.py ............
sympy/printing/tests/test_python.py .....x....
sympy/printing/tests/test_repr.py ....................
sympy/printing/tests/test_str.py ........................................................................
sympy/printing/tests/test_tableform.py ..
sympy/printing/tests/test_theanocode.py ssssssssssssssssssssssssssss

============= 407 passed, 120 skipped, 4 xfailed in 52.75 seconds ==============
```