oxli_exception_convert.hh doesn't get installed

[camillescott]

The oxli_exception_convert.hh header doesn't get installed to a discoverable location with the .pxd files, making it not possible to cimport the new Cython bindings.

[camillescott]

We should probably come up with a test for linking liboxli and cimporting now that it's a more standard feature.