add some TODO-reminder && remove winxp-compatible code off file_enumerator
modified file_util && path_service

1.+ MakeAbsoluteFilePath function
2.* ensure every path returned by PathService::Get is an absolute path
modified file_util

add some function declarations and PathExists implementations
update README && modified to pickle

replace exhaustive interface with a bunch of overloaded interfaces
modified to pickle

add Pickle::empty() && change some functions access level
modified in pickle

1.Remove default-ctor of PickleIterator
2.update comments
add unittest for pickle (not yet finished)
add move-op for Pickle
modified in pickle && done with pickle unittest

done with move-ops for pickle
spell-correction in README && add doc for pickle && code format in pickle