Fix setup.py detail headers and add pip install tests to Travis

The default `install_headers` from `distutils` flattens all the headers
into a single directory -- `detail` subdirectory was lost. This commit
fixes this by overriding the setup with a custom header installer.

Tests are added to Travis to make sure `setup.py sdist` and `pip install`
do not miss any headers and that the directory structure is preserved.

[skip appveyor]