Video concatenation (#14)

* Simplify Strategies to return TrackStatus

* Comment sources

* Refactor strategies to accept more than one input format

* Change algorithm for merging input sizes

* Small changes

* Add multiple DataSource APIs, rename iFrameInterval to keyFrameInterval

* Add getLastTimestampUs to sources

* Complete multiple sources implementation

* Fix bugs

* Remove sampleRate exception

* Rotate through OpenGL instead of metadata

* Use DataSource getReadUs

* Add documentation