CLN/INT: rename *possibly* -> *maybe* functions
we have a mixed bag on naming this. Let's change ``possibly`` -> ``maybe``. Keep everything else the same. (if you do the same search for ``maybe`` you will fine it to be far more common). Note that the *usage* of these functions will have to be renamed as well :>

```
(pandas) bash-3.2$ grep --include '*.py'  -R possibly pandas |grep def
pandas/computation/expr.py:    def _possibly_transform_eq_ne(self, node, left=None, right=None):
pandas/computation/expr.py:    def _possibly_downcast_constants(self, left, right):
pandas/computation/expr.py:    def _possibly_eval(self, binop, eval_in_python):
pandas/computation/expr.py:    def _possibly_evaluate_binop(self, op, op_class, lhs, rhs,
pandas/core/internals.py:def _possibly_compare(a, b, op):
pandas/indexes/base.py:    def _possibly_promote(self, other):
pandas/tests/types/test_cast.py:    def test_possibly_convert_objects_copy(self):
pandas/tseries/index.py:    def _possibly_promote(self, other):
pandas/tseries/tdi.py:    def _possibly_promote(self, other):
pandas/types/cast.py:def _possibly_convert_platform(values):
pandas/types/cast.py:def _possibly_downcast_to_dtype(result, dtype):
pandas/types/cast.py:def _possibly_cast_item(obj, item, dtype):
pandas/types/cast.py:def _possibly_convert_objects(values, convert_dates=True, convert_numeric=True,
pandas/types/cast.py:def _possibly_castable(arr):
pandas/types/cast.py:def _possibly_infer_to_datetimelike(value, convert_dates=False):
pandas/types/cast.py:def _possibly_cast_to_datetime(value, dtype, errors='raise'):
```