Update cmake conf for testing.
Suppress error from signed comparison in gtest.h (-Wno-error=sign-compare).
Suppress error from unused variables (-Wno-error=unused-but-set-variable).
Add cmake flag BUILD_TESTING to the cached variables.
When building for testing, the include path to GTest shall be added.
Enable testing for Linux with gcc.
Resolve namespace std clash with boost.
struct NullDeleter can't be local.
Remove surplus ';'.
Minor readme updates.

Add link to define POD.
Refer to entity instead of component.
Merge pull request #1 from larspensjo/linux-gcc

Enable testing for Linux GCC.