Consistently use Gremlin.Net instead of Gremlin-DotNet

We agreed in TINKERPOP-1552 to name the .NET GLV and driver Gremlin.Net, but I used Gremlin-DotNet instead in the initial pull request. The documentation still uses mainly Gremlin-DotNet which could be a bit confusing to new users. So we should simply replace Gremlin-DotNet everywhere by Gremlin.Net.