Release 0.0.5

Fix compatibility with OpenTracing-api version 0.22.0.

Bump OpenTracing API to 0.21.0

This release contains:

• New OpenTracing API version 0.21.0
• Removal of SpanPropagatingExecutors class (#11)
• JavaDoc fixes

Rename manage/release to activate/deactivate.

This release is a result of PR #8 in which the manage(Span) operation was renamed to activate(Span)
and the ManagedSpan.release() was renamed to ManagedSpan.deactivate().

Documentation has been updated, but where a reference was missed, please feel free to file an issue.