Move header files to an include directory

[gummif]

It is standard practice to have exported headers in a directory called include. I would have liked to move the code to include/cppzmq but that would be a breaking change forcing code to do #include <cppzmq/zmq.hpp> (maybe for the next major release?).

[coveralls]

Pull Request Test Coverage Report for Build 266

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 85.073%

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Totals
Change from base Build 264:	-0.5%
Covered Lines:	644
Relevant Lines:	757

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💛 - Coveralls

[bluca]

please don't, this will break a lot of scripts and packages

[gummif]

@bluca Yes it will for non-cmake consumers. Would it make you more comfortable if this change were made in a major release (v5.0)?

[bluca]

Sorry, but I don't feel like a cosmetic change on the repository layout is a good enough reason to break compatibility for all distros, regardless of versioning

[sigiesec]

While I don't particularly like the current layout either, I agree with @bluca here. We only have two header files, so the situation on our side is not bad enough to warrant all the troubles a layout change might cause.

[gummif]

I understand the concerns and this PR can be closed. Although I disagree that code layout change are unacceptable at the same time breaking code changes are introduced with a major version. I guess that's a conflict between long term backwards compatibility and using new features and best practices.

[bluca]

Breaking backward compatibility can happen and the process is defined in C4 - but given the huge inconvenience, it should balanced by a very compelling requirement