Pyhmmer

Added
- `TextSequence` and `DigitalSequence` representing a `Sequence` in a given mode.
- E-value properties to `Hit` and `Domain`.
- `TopHits` now stores a reference to the pipeline it was obtained from.
- `Pipeline.Z` and `Pipeline.domZ` properties.
- Experimental pickling support to `Alphabet`.
- Experimental freelist to `Sequence` class to avoid allocation bottlenecks when iterating on a `SequenceFile` without recycling sequence buffers.

Changed
- Made `Sequence` an abstract base class.
- Additional `Pipeline` parameters can be passed as keyword arguments to `pyhmmer.hmmsearch`.
- `SequenceFile.read` can now be configured to skip reading the metadata or the content of a sequence.

Removed
- Redundant `SequenceFile` methods.

Fixed
- `doctest` loader crashing on Python 3.5.
- `TopHits.threshold` segfaulting when being called without prior `Tophits.sort` call
- Unknown `format` argument to `SequenceFile` constructor not raising the right error.

Added
- Support for compilation on PowerPC big-endian platforms.
- Type annotations and stub files for Cython modules.

Changed
- [`distutils`](https://docs.python.org/3/library/distutils.html) is now used to compile the package, instead of calling `autotools` and letting HMMER configure itself.
- `Bitfield.count` now allows passing an argument (for compatibility with [`collections.abc.Sequence`](https://docs.python.org/3/library/collections.abc.html#collections.abc.Sequence)).

Initial alpha release (test deployment to PyPI).