Explicit column dtype specification in read_* functions
e.g. columns with values like `01001` are getting converted to int

example from mailing list:

```
df = read_csv('test_data.csv')
df.head()
     oid   did mode             ox             oy      dx      dy
0  1001  1001   01  272311.659358  176751.822655  272675  176375
1  1001  1001   01  272311.659358  176751.822655  272375  176375
2  1001  1001   01  272311.659358  176751.822655  272125  176675
3  1001  1001   06  272311.659358  176751.822655  272675  177125
4  1001  1001   06  272311.659358  176751.822655  272675  176375

df.oid = df.oid.apply(lambda x: str(x).zfill(5))
df.head()
     oid   did mode             ox             oy      dx      dy
0  01001  1001   01  272311.659358  176751.822655  272675  176375
1  01001  1001   01  272311.659358  176751.822655  272375  176375
2  01001  1001   01  272311.659358  176751.822655  272125  176675
3  01001  1001   06  272311.659358  176751.822655  272675  177125
4  01001  1001   06  272311.659358  176751.822655  272675  176375
```
