FIX: Bug whereby array_equivalent was not correctly comparing Float64Ind...
Currently, 

```
>>> import pandas.core.common as com
>>> com.array_equivalent(Float64Index([0, np.nan]), Float64Index([0, np.nan]))
False
```

Although the current pandas code base does not use `array_equivalent` to compare Float64Indexes, leaving `array_equivalent` in its current state may be a bug waiting to happen. 

This PR attempts to fix the problem by using `pd.isnull` for all arrays of dtype `object`. In a previous PR I tried this and got terrible perf results. Since then I've discovered that my machine does not have enough memory to run the full perf test suit without page faults. If I rerun `test_perf.sh` for just a few Benchmarks, I can avoid the page faults and get consistent results.

Running `/usr/bin/time -v ./test_perf.sh -b master -t fix-equivalent` yielded two tests with ratio > 1.1. 

``` -------------------------------------------------------------------------------
reindex_fillna_pad                           |   0.5784 |   0.5034 |   1.1490 |
packers_write_pack                           |  15.2360 |   7.1851 |   2.1205 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```

which I believe were due to page faults. When I reran perf on just these tests using
`/usr/bin/time -v ./test_perf.sh -b master -t fix-equivalent  -r "reindex_fillna_pad|packers_write_pack"`

I got

``` -------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
reindex_fillna_pad_float32                   |   0.4633 |   0.4590 |   1.0093 |
packers_write_pack                           |   7.9544 |   7.8390 |   1.0147 |
reindex_fillna_pad                           |   0.7290 |   0.7180 |   1.0154 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```
