Taxtastic

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* remove support for reading excel spreadsheets (GH-71)
* requirements.txt identifies all direct dependencies
* add ``taxit merge_taxtables``
* New ``taxit new_database --taxdump`` and ``taxit new_database--taxdump-url`` arguments
for flexibility on taxdump.zip location(s)
* New function taxtable.remove_subtree() (GH-80)

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* new ``taxit count_taxids`` counts every tax_id occurance in a ``taxit taxtable`` lineage [GH-75]
* new ``taxit taxid_classified`` decides if a tax_id is primary and valid (True/False)
* ``taxit update_taxids`` will halt on unknown tax_ids unless ``--unknowns FILE`` is specified
* ``taxit update_taxids`` only requires a csv file with 'tax_id' column
* ``taxit update_taxids`` takes an optional ``--name-column`` to assist in assigning tax_ids
* ``taxit update_taxids`` will read stdin if csv file is not provided as argument

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* Add ``taxit taxtable --full`` for outputing all ranks in header.
* Update subcommand help text
* Generate Sphinx docs using help text emitted by subcommands (GH-70)

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* Suppress warning when updating refpkg ``tree_stats`` file via ``taxit update``.

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* Fix GH-63: "empirical_frequencies" now set to false when parsing FastTree AA statistics files
* Close GH-64: "empirical_frequencies" is now available as a flag for PhyML statistics files
* Fixed bug that prevented temporary files from being deleted

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* Fix GH-62: "empirical_frequencies" was not set when parsing PhyML AA statistics files.