Sort headers
Use opaque classes to remove dependencies

This potentially speeds up compilation
Make the header being implement always the first
Merge pull request #7 from jeroenk/header-reorg

Reorganisation of the header inclusion
Updated .gitignore
Moved quadruple to Util.
Some "include what you use".
Fix typo
Merge pull request #10 from jeroenk/typo

Fix typo
Moved quadruple.h to the right library.
Revert "This should be contained in "llvm-config --cxxflags" if needed."

This reverts commit 5e56bedd266e41f5b9afd918aefbf14e0cccb36e.