runtests: Show operands of failed tests' assert statements

This uses ast module (if available) to rewrite assert statements of the form:

```
assert expr_0 cmpop_0 expr_1 cmpop_1 ... cmpop_n expr_{n+1}
```

to:

```
(sym_0, sym_1, ..., sym_{n+1}) = (expr_0, expr_1, ..., expr_{n+1})
assert sym_0 cmpop_0 sym_1 cmpop_1 ... cmpop_n sym_{n+1},
    "%s cmpop_0 %s ... cmpop_n %s" % (sym_0, sym_1, ..., sym_{n+1})
```

Line numbers are preserved on assert statements. On short test runs there is a
small but visible speed penalty (e.g. under 10% for sympy/polys). However, full
test suit run isn't affected that much.

Plain bin/test:

```
$ bin/test sympy/polys/tests/test_rings.py
============================================================ test process starts ============================================================
executable:         /usr/bin/python  (2.7.2-final-0)
architecture:       64-bit
cache:              yes
ground types:       gmpy 1.14
random seed:        52276056
hash randomization: on (PYTHONHASHSEED=2961690477)

sympy/polys/tests/test_rings.py[12] .......F....                                                                                       [FAIL]

_____________________________________________________________________________________________________________________________________________
___________________________________________ sympy/polys/tests/test_rings.py:test_PolyElement_pow ____________________________________________
  File "/home/mateusz/repo/git/sympy/sympy/polys/tests/test_rings.py", line 78, in test_PolyElement_pow
    assert f**0 == 3
AssertionError

=========================================== tests finished: 11 passed, 1 failed, in 0.03 seconds ============================================
DO *NOT* COMMIT!
```

Enhanced bin/test:

```
$ bin/test sympy/polys/tests/test_rings.py -E
============================================================ test process starts ============================================================
executable:         /usr/bin/python  (2.7.2-final-0)
architecture:       64-bit
cache:              yes
ground types:       gmpy 1.14
random seed:        57370498
hash randomization: on (PYTHONHASHSEED=4164250690)

sympy/polys/tests/test_rings.py[12] .......F....                                                                                       [FAIL]

_____________________________________________________________________________________________________________________________________________
___________________________________________ sympy/polys/tests/test_rings.py:test_PolyElement_pow ____________________________________________
  File "/home/mateusz/repo/git/sympy/sympy/polys/tests/test_rings.py", line 78, in test_PolyElement_pow
    assert f**0 == 3
AssertionError: 1 == 3

=========================================== tests finished: 11 passed, 1 failed, in 0.06 seconds ============================================
DO *NOT* COMMIT!
```