ASR API enhancements

[ar13pit]

• Moving saver from audio source to ASR. We have an api to get chunks. Let the module connecting to the source (i.e. ASR) decide whether saving is required
• Let ASR api start and stop stream, instead of doing it explicitly as it serves no particular purpose outside the ASR environment
• Unify Source, ASR, Sink base classes as a Pipeline base class
• Provide an interface to allow pipeline elements to connect to each other
• Define a pipeline class that takes in pipeline elements and controls the data flow from source to sink. (Unidirectional information flow)
• Remove calls to source and sink methods in elements. Let pipeline class handle the movement of data between different elements.

Work being done in branch feature/split_sink and PR #5

[ar13pit]

• Better file handling in WaveFileSink so that files are not overwritten and preferably support filename formats (this will be needed with the ros wrapper)
• Add support for writing out the decoded string (this would require the definition of a chunk data structure that is passed along the entire pipeline)
• Add more continuity checks in the AsrPipeline class
• Document the mechanism of pipeline