Simplify + clarify usage

[rhaschke]

As we are planning to use this package for MoveIt's Travis checks (https://github.com/ros-planning/moveit_ci/pull/91), I would like to incorporate some simplifications, fixes, and clarifications before:

• include(CodeCoverage) shouldn't be required explicitly. Rather these definitions should be included via find_package() already.
• Defining the option ENABLE_COVERAGE_TESTING in the package's cmake file doesn't make sense. It needs to be defined in the module included by downstream packages.
• The README should clarify that this package provides two things:
  • It allows the instrumentation of the generated code for coverage reporting - via APPEND_COVERAGE_COMPILER_FLAGS()
  • It allows to generate the html coverage report - via add_code_coverage()

@mikeferguson, would be great, if you could create a new release into Kinetic and Melodic if this has been merged.

[mikeferguson]

@rhaschke Thanks! All good ideas. 0.3.0 release is going out to kinetic/melodic now