static code analysis: introduce automated scans via SonarCloud

This adds Travis and Docker configurations for automated scans,
both for master branch and internal PRs.

External PRs won't be checked due to security concerns
(and Travis limitation related to that) - GH and SonarCloud tokens
are not propagated to those.

An organization and project must be created in SonarCloud for reporting,
as well as a technical GH user with mraa repo commit permission, to set
PR statuses in the "checks" section.

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>

.travis.yml

@@ -23,6 +23,7 @@ env:
   - TARGET=java BUILDARCH=MOCK
   - TARGET=node4 BUILDARCH=MOCK
   - TARGET=node5 BUILDARCH=MOCK
+  - TARGET=sonar-scan
 
 matrix:
   exclude:
@@ -32,6 +33,8 @@ matrix:
       env: TARGET=java JSONPLAT=ON
     - compiler: clang
       env: TARGET=java BUILDARCH=MOCK
+    - compiler: clang
+      env: TARGET=sonar-scan
   allow_failures:
     - compiler: gcc
       env: TARGET=node4 JSONPLAT=ON