Update JsonMapperTest.cs, changed ExportValueTypesTest, ImportValueTypesTest

ValueTypesTest has ulong type attribute, but it doesn't support ulong max value limit.
Update JsonReader.cs

For reading unsigned long (UInt64) type data, 
added some code to pass ValueTypesTest's ulong with max value.
Merge branch 'patch-1' of https://github.com/MickeyKim/litjson into MickeyKim-patch-1
Merge branch 'patch-2' of https://github.com/MickeyKim/litjson into MickeyKim-patch-1
Getting ready pull requests #13 and #14
Merge branch 'MickeyKim-patch-1'