remove 1 char line terminator limit
This PR closes #3501
This restriction is preventing the loading of CRLF delimited CSVs


Steps to reproduce:

```
import pandas as pd
import StringIO
pd.read_csv(StringIO.StringIO(' a,b,c\r\n'), header=None, lineterminator='\r\n')

ValueError: Only length-1 line terminators supported

```

while the following parses the CSV successfully: 

```
import pandas as pd
import StringIO
pd.read_csv(StringIO.StringIO(' a,b,c\r\n'), header=None)
```

