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Also, include one known difference to bind9. This is both to store it
somewhere before we forget about it and to have an example to how the
file would look like.

- Added make devel target in doc directory.
- Doxygen warnings/errors are now part of the Developer's guide itself.
- renamed mainpage.dox file
- Added extra information to main page
- Extended libdhcp++ descriptions
- Added doc/.gitignore to ignore doc/html (potentially 1000s of files)