Abutils

* updated ``abutils.io.read_csv`` to use ``pandas.read_csv`` rather than ``csv.DictReader``. This should result in better typing of numeric data fields.
* switched back to ``mmseqs`` for clustering.

Features:
* add translation function to abutils.core.sequence
* Sequence objects now have a translate() method

Other:
* Fix TravisCI link in README.md
* remove requirements2.txt, which only existed for Python 2.x compatibility

Updates
* update abutils.cluster.cluster() to use vsearch instead of mmseqs
* add read_sequences() function to abutils.io
* update abutils.pair.Pair objects to handle TCRs

Also, general bugfixes and updates to CI

Minor bugfixes, as well as a few small feature additions:

- add a ``read_csv()`` function to ``abutils.core.sequence``
- refactor the ``abutils.core.pair.Pair`` object:
- new ``heavies`` and ``lights`` properties
- ``vrc01_like`` is a standalone function in ``abutils.core.pair`` rather than a method of ``Pair``
- ``vrc01_like()`` now has an additional argument: ``loose``. If ``True``, a ``Pair`` will be considered VRC01-like if any of the chains in ``heavies`` or ``lights`` meet the VRC01-like criteria. If ``False``, the primary ``heavy`` and ``light`` chains must meet the criteria.

update api to include tl, pl and io
bugfixes

add and update abutils.tenx plotting functions
bugfixes