improved things and getActivities()

- added some parameters to getActivities:
getActivities($week=null, $month=null, $year=null)
Returns every activity in your Runtastic account as an array of objects.
If
  - `$iWeek` is set, all activities within requested week will be
returned (week starts on monday).
  - `$iMonth` is set, all activities within requested month will be
returned.
  - `$iYear` is set, all activities within the requested year will be
returned.

`$iWeek` and `$iMonth` can be used together with `$iYear`. if `$iYear`
is null, the current year will be used for filtering.

- Multiple requests are now possible.
- Removed some lines.
- Improved some code.