remove FilePath func, now lives on FileStorage
remove Entry

the details of persistence are only of value in the context of the
storage, no need to leak this information to the outside
instead of string, use TimeEntry internally

the change bears the overhead of having to parse each time entry even
though one might now work with it, but it unifies the assumption the
code has about interfaces etc
unify the specs using table driven tests
add simple timezone related spec
add a completly contrieved timezone example
change SummarizeDate to return a map[string]int64

this makes testing easier as well as another simplifying step in the
overall CLI structure