All multiple string concatenations using "+" changed to string.Concat.
RegexConverter.WriteBson, JsonSerializerInternalBase.IsErrorHandled, JsonSerializerInternalWriter.CheckForCircularReference  and JsonPosition.FormatMessage implementations changed to use StringBuilder.
JToken.Path property implementation improved by using AppendFormat instead of 3 string concatenations.
MemoryTraceWriter.Trace improved Format method usage.
Added some extension methods to work with StringBuilder: Trim and EndsWith. Code was taken from my project https://github.com/xperiandri/StringBuilderExtensions
Merge remote-tracking branch 'origin/master'

Conflicts:
	Src/Newtonsoft.Json/Bson/BsonReader.cs
	Src/Newtonsoft.Json/Converters/RegexConverter.cs
	Src/Newtonsoft.Json/Converters/XmlNodeConverter.cs
	Src/Newtonsoft.Json/JsonPosition.cs
	Src/Newtonsoft.Json/JsonSerializer.cs
	Src/Newtonsoft.Json/Linq/JToken.cs
	Src/Newtonsoft.Json/Serialization/JsonSerializerInternalBase.cs
	Src/Newtonsoft.Json/Serialization/JsonSerializerInternalWriter.cs
	Src/Newtonsoft.Json/Serialization/MemoryTraceWriter.cs
	Src/Newtonsoft.Json/Utilities/DynamicWrapper.cs
	Src/Newtonsoft.Json/Utilities/EnumUtils.cs
	Src/Newtonsoft.Json/Utilities/MiscellaneousUtils.cs