Interpolate fails when passed limit argument.
When I try to interpolate time series data it fails with an index error: arrays used as indices must be of integer (or boolean) type.

``` python
from pandas import read_csv
from StringIO import StringIO

data = '''2013-09-28,4018.02
2013-10-05,5806.78
2013-10-26,6334.4
2013-11-02,6199.6
2013-11-09,6213.07
2013-11-16,7099.39
2013-11-23,7332.98
2013-11-30,6157.37
2013-12-07,7427.65
2014-01-04,5856.38
2014-01-11,6208.99
2014-01-18,7440.29
2014-01-25,7295.97
2014-02-08,7654.42
'''

df = read_csv(StringIO(data))
df.interpolate(limit=1)
```

Removing the `limit=1` argument works as expected.

Python 2.7.6, pandas 0.14.0, numpy 1.8.1, on OS X.

I'll try it with a Python 3 environment when I get chance later this week.
