fixed namespace pollution by removing 'using' statements from headers

- including 'using namespace XXX' in the header causes this to be effective for every file including the header, potentially resulting in namespace pollution and conflicts
- 'using ...' should only be used in the implementation where it is not potentially affecting other parts of the code
Merge remote-tracking branch 'upstream/master' into namespace_cleanup
Merge pull request #53 from hperrey/namespace_cleanup

Avoid namespace pollution through header files which have 'using' statements