C++ cleanups and activation of continuous integration

[michaelzs85]

This pull request contains refactoring and addition of a .travis.yml file but no functional changes.

• Fixed some problems where the library wouldn't compile with -Werror activated (e.g. removed some unnecessary typedefs)
• removed braces from return statements as they are unnecessary
• used = default for empty destructors, removed unnecessary void as function parameters
• refactored variable declarations to happen at initialization. (See CPPCoreGuidelines ES.22) (declaring them early inhibits optimizations)
• made methods constwhere possible
• added a .travis.yml file to activate continuous integration
  • If you haven't worked with travis-ci before go to travis-ci.com and log-in with your git-hub account and follow the first steps.
  • For every push a CI-job will be started. All the sketches in the examples folder will be build.

[shurillu]

Thank you man!