HEALPix include files moved to Healpix_cxx/ dir

The new autotools-based HEALPix packages install headers to
${prefix}/include/Healpix_cxx to avoid collisions with other files.
Link against healpix_cxx only

In the new Autotools-based HEALPix packages, all symbols are in just
this one library.
Use pkg-config to locate external HEALPix

The `build_healpix` subcommand has morphed into `build_external_clib`,
which now supports locating any externally installed package with
`pkg-config`, falling back to building from bundled source.

To add another `pkg-config` style package, add a new entry to the
`libraries` argument to `setup()`, that looks like this:

    ('lib_name', {
        'pkg_config_name': 'name_of_pc_file',
        'local_source': 'path_to_bundled_source_distribution'})

As an example:

    ...
    libraries=[
        ('cfitsio', {
            'pkg_config_name': 'cfitsio',
            'local_source': 'ext/cfitsio'})],
    ...
Override build_clib.get_source_files

The overriding method makes it so that it is not an error for a
build_info dictionary to lack a 'sources' key. This is necessary to
support the 'sdist' subcommand, which calls build_clib.get_source_files.
Initialize build_args instance variable in __init__

This is just better style.
Update healpixmirror submodule
Determine which bundled sources to distribute

The build_external_clib subcommand now automatically adds any sources
for bundled libraries to the distribution. It is no longer necessary to
name such sources explicitly in MANIFEST.in.
Run 'autoreconf -i' for any bundled libraries

For autotools-based packages, this generates the 'configure' script if
it does not already exist.
Optionally build against bundled cfitsio

cfitsio does not support building outside of the source directory, so
this commit also adds an optional 'supports_non_srcdir_builds' key to
build_info that instructs build_external_clib to copy the source into
the temporary build directory.
Add internally built pkgconfig directory to PKG_CONFIG_PATH

As a result, as external libraries are built, they get picked up by
subsequent libraries that depend upon them.
Updated documentation to reflect new build with pkg-config
Do not pass output= argument to CalledProcessError

The output= argument was added in Python 2.7 to support the
subprocess.check_output function, so if we do need to fall back and
provide check_output, we cannot pass the output= argument.
Remove unused import
Make pkg-config name for healpix_cxx lower case
Add Cython-generated .cpp files to MANIFEST.in

This has the effect of including them in tarballs generated by
'python setup.py sdist'.
Add library paths to runtime_library_dirs

This ensures that shared libraries are in the rpath, so that they do not
have to reside in the default shared linker path.
Add external library build arguments to individual extensions

...rather than to the build_ext instance.
travis-ci config file
Remove duplicate import
Remove unused import