Fixed nunit.exe.config to support setting breakpoints when running unit tests.
CSHARP-509: Remove superfluous Create methods.
CSHARP-509: Change how BsonValue Create methods handle C# null.
CSHARP-509: BsonDocument object model Add methods now throw ArgumentNullException if called with a C# null argument.
CSHARP-509: Fixed unit tests for Add and AddRange with C# null.
CSHARP-509: Check for null keys.
CSHARP-555: Throw if Merge is called with null argument. And fix failing unit tests that weren't detected in the last commit because the NUnit Visual Studio extension got messed up and wasn't running all the tests.
CSHARP-555: Fix some more failing unit tests that the weren't detected for some unknown reason by the NUnit Visual Studio plugin.