New formula to set edge radius according to satellite height.

Small suggestion fixes problem mentioned in target branch
 - see https://github.com/SciTools/cartopy/pull/264

Problem to calculate the right value of _max based on the satellite height.

Tested like this...
```
import matplotlib.pyplot as plt
import cartopy.crs as ccrs

a = 6378137.0
plt.figure(figsize=(20,10))
for i, scale in enumerate([16.0, 8.0, 4.0, 2.0, 1.0, 0.5, 0.25, 0.125]):
    ax = plt.subplot(2, 4, i+1, projection=ccrs.Geostationary(satellite_height=scale*a, central_longitude=120.0))
    ax.coastlines()
plt.show()
```

Checked behaviour with modified code in Geostationary :  If you get set the _max value wrong, the coastline lines will either leave a gap at edge, or overlap edge on plots.
Whereas this new version seems to get it right.