Add --rerun option (and counter) to bin/doctest.

Assures a consistent API for test and doctest.  Suggested [here] (https://github.com/sympy/sympy/pull/8054#issuecomment-56190579)

Some examples:

* Regular behavior when `--rerun` not specified
```bash
$ bin/doctest sympy/core/cache.py
============================= test process starts ==============================
executable:         /home/peter/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
hash randomization: on (PYTHONHASHSEED=2427431791)

sympy/core/cache.py[1] .                                                    [OK]

================== tests finished: 1 passed, in 0.10 seconds ===================
```

* With `--rerun`
```bash
$ bin/doctest sympy/stats/crv_types.py --rerun 3
============================= test process starts ==============================
executable:         /home/peter/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
hash randomization: on (PYTHONHASHSEED=288382127)

sympy/stats/crv_types.py[35] ...................................            [OK]

================= tests finished: 35 passed, in 17.04 seconds ==================
rerun 1
============================= test process starts ==============================
executable:         /home/peter/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
hash randomization: on (PYTHONHASHSEED=973603448)

sympy/stats/crv_types.py[35] ...................................            [OK]

================= tests finished: 35 passed, in 17.13 seconds ==================
rerun 2
============================= test process starts ==============================
executable:         /home/peter/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
hash randomization: on (PYTHONHASHSEED=3823669305)

sympy/stats/crv_types.py[35] ...................................            [OK]

================= tests finished: 35 passed, in 17.77 seconds ==================
rerun 3
============================= test process starts ==============================
executable:         /home/peter/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
hash randomization: on (PYTHONHASHSEED=3696587817)

sympy/stats/crv_types.py[35] ...................................            [OK]

================= tests finished: 35 passed, in 17.41 seconds ==================
```
I changed `_test` to `_doctest` here. I missed that in review. Sorry.