Change LogFormatterExponent to consistently format negative exponents

Currently, using `LogFormatterExponent` results in inconsistently formatted labels for values less than one.  Integer values greater than one are displayed as integers, while values below one are displayed using scientific notation.

For example, the following currently produces an unexpected result:

```
import matplotlib.pyplot as plt
from matplotlib.ticker import LogFormatterExponent

fig, ax = plt.subplots()
ax.loglog()
ax.axis([10**-5, 10**5, 10**-5, 10**5])
ax.xaxis.set_major_formatter(LogFormatterExponent())
plt.show()
```
![figure_1](https://f.cloud.github.com/assets/906803/1795214/6a7b22dc-69f4-11e3-9e23-303996095fc4.png)

---

With this fix, values below one are consistently formatted:

![figure_1](https://f.cloud.github.com/assets/906803/1795223/c584fd1e-69f5-11e3-9e22-3976d2c3f7c7.png)


This change does arguably break backwards compatibility, but I think (?) the current behavior is unintentional.