Replaced IMapEntry and MapEntry with KeyValuePair.
Fixed bugs where a method was supposed to return null when a key/index was not found,
but couldn't because the generic was not a class but a primitive type or a struct.
Split unit tests up into different files.
Updated documention in IPersistentList to C# style.
Updated documentation in IPersistentMap to C# style.
Updated documentation in IPersistentVector to C# style.
Fixed bugs to do with peeking and poping empty vectors and lists.
Prepared release for next nuget package (v0.2.0)
Updated readme file with release notes.