RF: add pip arguments to command parser, test

Add some pip input arguments to the command parser, and test these
behave as expected when run through pip argument extracter.

This breaks the command for now.
RF: add positional requirement args to pip args

Positional requirements args might also be passed to pip, but also allow
positional args, requirement files not to be returned in
``recon_pip_args`` so we can just get fetching-type flags.
RF: rename requirement specifiers variable

To make clear the difference between a requirement specifier and a
requirement file.
RF: allow main() argument to return exit value

Use return argument of main() in command definition to give exit value
from script.
RF: remove unused old copy of pip param getter

Current version is ``recon_pip_args`` in piputils.py
RF: check that at least one requirement given

Now we can use requirement files, requirements can come from positional
arguments or requirement files.  We need to check for case where neither
have been given.
RF: rename build directory for disk image

``dmg-build-dir`` seemed a bit clearer than ``dmg-in-dir``.
RF: return pip argments in two lists

Return pip arguments split into requirement arguments and other
arguments.  This makes it easier to use the fetching arguments for the
pip command when not installing these given requirements.
RF: refactor command to use pip parsing

Remove get_wheels because it's now just a one-liner.
BF: call test command through python binary

Script now does not shebang properly, with ``#!python`` shebang line
needed for proper wheel installation.  Call script via ``python``.
RF: allow None as requirement files argument

None instead of empty sequence
TST: add import check after running installer

Add import check at end of ``install`` Makefile target.
RF: allow uninstall to fail in Makefile prep
NF: get_pip_pip can use paths with tilde

Allow get_pip_pip to have tilde for home directory. Add tests.
DOC: fix up usage string for main command
NF: jinja templating to write requirements

Use jinja2 templating to write a requirements file.
RF: refactor postinstall to use templating

Also - use requirements file in packages directory for install.
NF+TST: allow inserting / deleting template paths

Wrap module global in convenience functions.
NF: add replacement template directory option

Add option to use alternative template directory.
RF+TST: add basic script tests from delocate

Copy over delocate script test module, revert to using simple sys.exit
from main() function.

Add very small tests of wheels2dmg script.