Improve Series.interpolate to use index values
http://stackoverflow.com/questions/10484424/possible-bug-in-series-interpolate

```
x1 = np.array([0, 0.25, 0.77, 1.2, 1.4, 2.6, 3.1])
y1 = np.array([0, 1.1, 0.5, 1.5, 1.2, 2.1, 2.4])
x2 = np.array([0, 0.25, 0.66, 1.0, 1.2, 1.4, 3.1])
y2 = np.array([0, 0.2, 0.8, 1.1, 2.2, 0.1, 2.4])

df1 = DataFrame(data=y1, index=x1, columns=['A'])
df1.plot(marker='o')

df2 = DataFrame(data=y2, index=x2, columns=['A'])
df2.plot(marker='o')

df3=df1 - df2
df3.plot(marker='o')
print df3

def resample(signals):
    aligned_x_vals = reduce(lambda s1, s2: s1.index.union(s2.index), signals)
    return map(lambda s: s.reindex(aligned_x_vals).apply(Series.interpolate), signals)

sig1, sig2 = resample([df1, df2])
sig3 = sig1 - sig2
plt.plot(df1.index, df1.values, marker='D')
plt.plot(sig1.index, sig1.values, marker='o')
plt.grid()
plt.figure()
plt.plot(df2.index, df2.values, marker='o')
plt.plot(sig2.index ,sig2.values, marker='o')
plt.grid()
```
