Add --split option to py.test

```bash
$ py.test sympy --split=1/10 -m 'not slow'
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.25 -- pytest-2.6.3
architecture: 64-bit
cache:        yes
ground types: python 

collected 6407 items 

sympy/assumptions/tests/test_assumptions_2.py .....
sympy/assumptions/tests/test_context.py ....
sympy/assumptions/tests/test_matrices.py ...x........x......
sympy/assumptions/tests/test_query.py ...............x............x..........................x
sympy/assumptions/tests/test_refine.py ......
sympy/calculus/tests/test_euler.py .....
sympy/calculus/tests/test_finite_diff.py ...
sympy/calculus/tests/test_singularities.py .x
sympy/categories/tests/test_baseclasses.py ...
sympy/categories/tests/test_drawing.py ........
sympy/combinatorics/tests/test_generators.py .
sympy/combinatorics/tests/test_graycode.py .
sympy/combinatorics/tests/test_group_constructs.py .
sympy/combinatorics/tests/test_named_groups.py .....
sympy/combinatorics/tests/test_partitions.py ...
sympy/combinatorics/tests/test_perm_groups.py ..............s..............s........
sympy/combinatorics/tests/test_permutations.py .......
sympy/combinatorics/tests/test_polyhedron.py .
sympy/combinatorics/tests/test_prufer.py ..
sympy/combinatorics/tests/test_subsets.py .
sympy/combinatorics/tests/test_tensor_can.py .........s..
sympy/combinatorics/tests/test_testutil.py .....
sympy/combinatorics/tests/test_util.py ........
sympy/concrete/tests/test_delta.py ..........................
sympy/concrete/tests/test_gosper.py ...........
sympy/concrete/tests/test_products.py ................
sympy/concrete/tests/test_sums_products.py ................................................
sympy/core/tests/test_args.py ...x.x..x.x.sss...........s...............s........s.ss....s........s..................s..s....s...............ss.........................................s..........................s...s........s.....s..sss.............ss..........................s......s...........ss..s...........s.....................s....s..s....s..xxxxxxxxxxx.s.s.......

==================== 1 tests deselected by "-m 'not slow'" =====================
 578 passed, 40 skipped, 1 deselected, 21 xfailed, 1 warnings in 112.81 seconds 
```

py.test does the splits on the number of tests rather than the number of files as is done in `bin/test`