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.
Add test for reusing an Entity.
This will lead to a crash.
Entity allocation error fixed.
Need to save the id before erasing.
Added comment.
Merge pull request #2 from larspensjo/ReuseEntity

Bug in allocation of freed entities.
Remove unused variable. Update README.