Improved tag support, refactoring
Improved tag support, refactoring

- Restore use of pointers for TimeEntry Start and Stop times; pointer
  use is needed for empty time values to be ommitted during JSON
  serialization. Add StartTime and StopTime functions to safely retrieve
  the start and stop time values.
- Add functions to create and delete tags
- Add function to add and remove tags from a time entry
- Add function to update an existing project
- Change instances of 'Timer' to 'TimeEntry'

jason0x43/go-toggl