Support out-of-source builds

[pcolby]

Since we're already fixing up the build process for #3 and #6, now's a good time to do it :smile:

[pcolby]

A work in progress; still need to:

• sort out the v4 test suite path;
• verify code coverage reporting still works;
• test other platforms.

[pcolby]

Also need to update the README.md file to describe the (recommended) out-of-source build steps.

[pcolby]

Also need to sort out all of the *.dox.in file escaping warnings from QMAKE_SUBSTITUTES.

[pcolby]

This should be all working now in Linux, OSX and Windows, except for the coverage reports (which I'll fix up next).

[pcolby]

All done, and merged to master now.