proj-datumgrid-1.7RC1 contains .gtx files too.

Make sure that the gie files in test/gie and test/gigs are included
in the distribution tarball. Tests with gie are set up to run all
available files matching *.gie in the gie and gigs directories. For
this reason tests that are known to fail in gigs/ have been renamed
to *.gie_failing, so that "make check" can be run in-tree in local
copies of the git repository.

Download grid files as the first thing in the travis install script and
that 'make check' can be run without errors caused by missing grids.

* Add ITRF init files, that makes it easy to use ITRF transformations in pipelines

* Added ITRF* files to the build systems

These files should be included in the distribution tarballs, but not installed by `make install`.

As discussed in the [Missing proj_def.dat in 4.9.1 release](http://lists.maptools.org/pipermail/proj/2015-September/007229.html) thead on the proj list,
`make dist` for proj shouldn't include the datumgrid files also included in the proj-datumgrid distributions.

The following files shouldn't be included in proj, because they're in proj-datumgrids already:

 * nad/ntf_r93.gsb        (included in proj-datumgrids <= 1.6RC1)
 * nad/ntv1_can.dat       (included in proj-datumgrids <= 1.6RC1)
 * nad/null.lla           (included in proj-datumgrids <= 1.5)
 * nad/nzgd2kgrid0005.gsb (included in proj-datumgrids <= 1.6RC1)

This change comprises two (partial) reverts:

 1) Revert "nad/Makefile.am: add missing 'null.lla ntf_r93.gsb nzgd2kgrid0005.gsb ntv1_can.dat' to EXTRA_DIST"

    This reverts commit a4c7892a94667e62d62caa053ac4b51ef790741c.

 2) Partial Revert "add files to EXTRA_DIST so they are removed by make uninstall"

    This partially reverts commit 0a7eda807da1e17f60558acebd89a08872b5b6e9.