Create* API

Right now the fitbit/client package only allows to GET data from the Fitbit API.

All the other methods haven't been implemented.

The implementation should be trivial. for example for implementing the Create Activity Goal one should:

1. Analyze the endpoint: there are 2 possible "periods", thus we can have 2 separate methods.
2. Is a "user" method (works on the authorized user).
3. It's an activity.
4. Therefore, in the fitbit/client folder we already have the user_activities.go file containing all the GET implementations, and this is the correct location for adding this method.
5. The convention to use is Create + User + $what. For example: CreateUserActivityDailyGoal and CreateUserActivityWeeklyGoal are the 2 methods to implement. NOTE: there are already the corresponding GET methods, therefore the naming should match (e.g. we have UserActivityDailyGoal for GET, therefore for the POST we must have CreateUserActivityDailyGoal
6. If the Response matches one of the already existing types (it should), then return this type. Otherwise, create a new one.