refactoring and usability fixes

a few big changes, hope these are ok:
- changed from everything using package 'main' to instead use 'fitbit' so it can be easily imported to other projects
- now using FITBIT_CLIENT_KEY and FITBIT_CLIENT_SECRET instead of hardcoding these values, this way others can easily run tests
- now reading tokens from ~/.fitbit-oauth file, if it does not exist, it goes through the oauth flow and then saves the tokens
- moved "tests" into proper testing namespace for building proper assertions in later
- added "example" directory for helping to generate the token as fmt.Scanln doesn't seem to work in tests so token should be generated first using this
- to do: update the readme, if this is accepted I can send another pull request with that update