* Create Audio Feature in SDK
* Add apis to record audio in SDK
* Add corresponding apis in pybind, storage.py, sdk.h
* Implement reservoir sampling when collecting audio samples

* fix clang format and update based on comment

* use int_8 for reading records and convert string directly from vector

* refract isSampleTaken to IndexOfSampleTaken

* fix clang format again

* Add manual Save feature.

* Fix the incorrect style

* Add feature to record Text

* Create a simple UI for text preview.

* Connect frontend with backend text data

* Move a constant from parameter to the body.
Remove test log

* Adjust the sync_cycle automatically.

* Change the multiplier type to double.

* Adjust the sync_period.

* Fix comment.

* Add the pre-commit

* Run pre-commit with other files.

* Only write the modified tablets to file system.
Move some functions definitions from headers to cc files.

* Persist the data in the destructor
Use unordered_set first.

* Use -1 for the unknown.

* Add C++ and python style check to travis.  
* Update all C++/python code that violate coding standards.