Inconsistency in CSV Import/Export
Pandas is not able to export a matrix as csv file and reimport it and keeping the data consistent. Using default arguments for `to_csv`, it will add an additional column for the indeces. Importing with default parameters will treat this column as data, not as index....

```
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.rand(10,10))
df.to_csv('np.csv')
print pd.read_csv('np.csv')


   Unnamed: 0         0         1         2         3         4         5  \
0           0  0.055663  0.492976  0.936424  0.931585  0.043748  0.931660   
1           1  0.946510  0.481707  0.935273  0.987895  0.982537  0.735273   
2           2  0.429818  0.090192  0.923747  0.973678  0.432166  0.318196   
3           3  0.579657  0.599554  0.794318  0.631867  0.700044  0.834421   
4           4  0.438074  0.747774  0.034653  0.113885  0.982059  0.736432   
5           5  0.379523  0.094214  0.435573  0.729742  0.778312  0.341792   
6           6  0.542644  0.175657  0.913459  0.532352  0.607791  0.369434   
7           7  0.132935  0.052179  0.145688  0.549158  0.127237  0.475737   
8           8  0.454960  0.872086  0.006616  0.444334  0.435469  0.435362   
9           9  0.141345  0.512531  0.900547  0.570482  0.366632  0.992289   

          6         7         8         9  
0  0.385482  0.432543  0.927187  0.408233  
1  0.385019  0.905481  0.852093  0.368507  
2  0.641478  0.966683  0.706884  0.229032  
3  0.592390  0.091528  0.969585  0.177480  
4  0.805170  0.585675  0.024259  0.961815  
5  0.818240  0.688166  0.175099  0.583955  
6  0.697869  0.202709  0.458018  0.546078  
7  0.597875  0.625422  0.055143  0.720858  
8  0.866318  0.348642  0.855215  0.689258  
9  0.723096  0.194654  0.681293  0.941478  
```
