multi-character line_terminator in to_csv can improperly convert scientific notation numbers to strings
Test case:

``` python
In [12]: test = pandas.DataFrame({"A": [1,2,3], "B": [1e-10,4.3434323e-8,3]})

In [13]: test.to_csv("test.txt", sep="\t", line_terminator="\n---\n")

In [14]: cat test.txt
        A       B

---
0       1       "1e-10"

---
1       2       "4.3434322999999997e-08"

---
2       3       3.0

---

```

Notice the quotes around the first two items in "B".
