Updating `test_seqsum.py`.

Updating reverse-complement error tests involving mafft. In these tests,
an alignment of primates is used where the human sequence is
intentionally reverse-complemented. `seqsum.summarize_distances` is
called on these sequences to ensure the rev-comp error is caught.
However, for some versions of mafft, the tarsius sequence was also
divergent enough to cause a rev-comp error when `ignore_gaps` was
`True`. Changing to `ignore_gaps` to `False` which causes the desired
testing behavior.
Updating `functions` module.

Adding `get_external_tool` and `get_too_full_path` functions, which
use `subprocess.Popen` to check the system for and executable.

Also, updating `is_executable` to use the new `get_external_tool`
function.

Lastly, updating `which` function to better handle full paths passed to
it.
Updating `argparse_utils.arg_is_executable`.

Updating this function to use the new `functions.get_external_tool`
function rather than `is_executable`. This allows names of executables
that are in the PATH to be used as an argument.
Updating version handling and incrementing version.

Adding package attributes (including __version__) to
``seqsift/__init__.py``. These attributes are now imported and used by
``setup.py``.

Incrementing package version to 0.1.1.

Incrementing seqvet.py version to 0.1.1.