Support for buffer contents

[agarny]

This PR adds support for buffer contents by adding the following methods:

• initializeFromBuffer();
• addFileFromBuffer(); and
• extractEntryToBuffer().

It also:

• renames initializeFromArchive() to initializeFromUnzipper() (for consistency);
• adds mapIterator() (for code reuse); and
• improves the build system (see issue zipper not found despite providing ZIPPER_LIBRARY and ZIPPER_INCLUDE_DIR #63 -- happy to create a separate PR for it, if really needed).

Fixes #61.
Fixes #63.

[fbergmann]

Thank you for the PR.

I still think, that the zipper issue should have been solved on the import target level (such that we won't require any of the variables in include_directory or worry about library names. But that can happen later.