add some comments
unnitest: add error_exception_util_unittest.cpp
new ReadMe
date_time: initial
add date_time

DateTime ctor(not finished)
date_time: DateTime ctors done
date_time modified:

1.add unittest for DateTime (not start coding yet)
2.add some interactive interfaces
modified registry & date_time & KBase_Test

1.format one line in registry.cpp
2.add conversion to other struct interface
3.add corresponding unittest
modified date_time

add overloaded comparison operators
date_time: add some format function declarations
modified date_time

precising underlying time_type and update corresponding interfaces
modified date_time

add impl for ToString/ToUTCString
modified date_time

1.change underlying data structure
2.add raw_time_since_epoch
add DateTimeSpan with fundamental functions
DateTimeSpan is done
modified to both DateTime and DateTimeSpan

1.swap the declaration order between DateTime and DateTimeSpan.
2.add add/subtract interface for time_type
3.DateTimeSpan conversion functon and arithmetics
4.DateTime arithmetic operators
replace raw SYSTEMTIME in file_enumerator with DateTime. also fix some flaws
add Arithmetic case