Prevent long terrain credits from being cut off

As discussed offline, terrain providers can currently only return
a single credit, therefore long credits get get off if the window
is to small or the credit to long.

The short term solution is to remove the `white-space: nowrap;` from
`cesium-credit-text` CSS class.  The longer solution is to allow
terrain providers to return multiple pre-broken up credits that can then
be laid out using nowrap.
Merge pull request #2011 from AnalyticalGraphicsInc/no-wrap

Prevent long terrain credits from being cut off