ethernetaddr.hh: adding all missed functions.

Issue #12.

ethernetaddr.cc module is not compiled in for now. Thus there are
possibilities to have undefined references. Moving all required methods
to header file and removing source module completely.
Small styling cleanups.
ethernetaddr.hh: moving hexit_value to unnamed ns.

Issue #12.

Addressing review comments from tootoonchian.
Merge pull request #13 from vitalivanov/issue-12

ethernetaddr.hh: make the library header-only and move helper functions to anonymous namespace.