FIX: Fixed some issues around vb_suite
I'm new to vb_suite, and ran following command according to [this document](https://github.com/pydata/pandas/wiki/Performance-Testing).

```
./test_perf.sh -b master -t HEAD
```

But I got some errors (`InvalidGitRepositoryError` and Benchmark duplication errors), and finally I got tests run by fixing following issues.
- VB_DIR pointed wrong directory
- temporary variable `bmark` in groupby was collected as Benchmark
- indexing module had same name ('series_getitem_scalar') Benchmarks

I'm not so familier with vb_suite, and any feedbacks are welcome ! Thanks.

was it hanging on something particular?

fyi, you can also run with `-r group` for example to only run some of the benches
 Sorry for mashing up multiple modifications in one pull request. Problems I got are like commented above. As to `indexing` module, I got unexpected result now, and so I'll investigate on it and comment about it later. **It seems that, if you already have a correct database, you doesn't get UNIQUE Constraint issue above.**

> fyi, you can also run with -r group for example to only run some of the benches

thanks.
 > As to indexing module, I got unexpected result now, and so I'll investigate on it and comment about it later.

Investigated, and added commit for that.
 @ledmonster thanks!
