make add_time entry use start_datetime and duration

New argument pattern: args should be: ENTRY [@PROJECT] START_DATE_TIME DURATION

__UNTESTED at this commit__

* changed the project check to be optional and look for argument starting with @ character. Used the same check from start_time_entry
* Read and parse the start date time argument
* after the entry JSON is created, update the start time
* updated the help actions and description in the function comments
Add completed time entries w/ duration or end time

Changed the final argument for add time entry.

If the final argument begins with d its treated as a duration.
Otherwise, its treated like an end time

I reordered the add_time_entry flow to first create a 0 duration time entry and then append the start and end_time/durations as needed.

Note, if an end time and duration are passed the duration needs to be the _actual_ duration (the delta in seconds between start and stop.
Removing debug strings
Fixes 14 - add time entry not working

Merge branch 'add_time_entry'
Merge branch 'master' of github.com:drobertadams/toggl-cli
Merge pull request #7 from beauraines/master

Brings add_time_entry forward to v8