Data keypairs bug fix

reproduce:

```python
>>> from vincent import Data
>>> Data.keypairs([1, 2, 3])
NameError: global name 'append' is not defined
```

This fixes it.

FYI:
1. the first commit already exists in my previous pull request. it would gone when that merged.
2. I done some pep8 coding style cleanups since it gives me many errors in my editor.
3. for the append part, I just removed it, the append param seem to come from here https://github.com/dnmiller/vincent/blob/d9bfd965e496759d2203ce6afca9e386b5a7a320/vincent/vincent.py#L409, and since the `if append` no longer exists, I assume it's no need.