GPII-343: Adding a Node.js bridge to call ALSA

[changes]
  * Adding both methods to get and set the master volume
  * Adding binding.gyp
  * Adding some tests at add-on level
GPII-343: Adding a Settings Handler for ALSA

[changes]
  * Adding package.json
  * Adding a Settings Handler which uses the Node.js ALSA volume bridge
  * Adding tests for the ALSA Settings Handler
GPII-343: Adding the new ALSA stuff into the GPII

[changes]
  * Add alsa into both build and clean processes
  * Add alsa into Linux's index.js in order to be loaded by the
framework at start time
GPII-343: In build.sh, update the comment for ALSA
GPII-343: Setting max and min values for volume range

This change will avoid errors while transforming values, no need to
round numbers, etc.
Merge remote-tracking branch 'javihernandez/GPII-343'