COMPAT: hashtable vectors depend on refcount semantics which do not work on PyPy
PyPy 5.7 can build and pip install pandas. Most of the tests pass, instructions to reproduce are [here](https://gist.github.com/mattip/64eabbc3c60b96e1bd1fd4751dfcc12c) However this code

```
import pandas; 
df = pandas.Series(['b','b','b','a','a','b']); 
print df.unique()
```
does not work on PyPy:

```
Traceback (most recent call last):
  File "<module>", line 1, in <module>
  File "pypy_stuff/pypy-latest/site-packages/pandas/core/series.py", line 1241, in unique
    result = super(Series, self).unique()
  File "pypy_stuff/pypy-latest/site-packages/pandas/core/base.py", line 973, in unique
    result = unique1d(values)
  File "pypy_stuff/pypy-latest/site-packages/pandas/core/nanops.py", line 811, in unique1d
    uniques = table.unique(_ensure_object(values))
  File "pandas/src/hashtable_class_helper.pxi", line 826, in pandas.hashtable.PyObjectHashTable.unique (pandas/hashtable.c:14521)
ValueError: cannot resize an array with refcheck=True on PyPy.
Use the resize function or refcheck=False

```

Calls to ``array.resize(..., refcheck=True)`` (note that refcheck=True is the default) check that data reallocation is needed, and if so checks that no other object is dependent on ``array`` by testing the ``array`` object's refcount. This check is unreliable on PyPy, since we do not use a reference counting garbage collector.

I started a branch to simply expose the ``refcheck`` keword through the various places needed, the commit can be seen on my fork of pandas [here](https://github.com/mattip/pandas/commit/56233e45544). 

The caller in this patch can know with certainty that there are no other users of the object, and so refcheck=False is safe. The changes are a bit intrusive, so I am opening this as an issue first hoping it generates some discussion before I issue a pull request.