Brie

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* move __version__ into a separate file, so import brie is not required before
installation.
* support cram file as input aligned reads

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* split the pre-processing functions in BRIE to another separate package
BRIE-kit, as some functions in the pre-processing require Python2 environment.
So, `BRIE` will keep two functions `brie` and `brie-diff`, while `BRIE-kit`
will have `briekit-event`, `briekit-event-filter`, and `briekit-factor`.
See BRIE-KIT: https://github.com/huangyh09/briekit/wiki

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* support gff in gz format
* add an example with 130 cells
* `brie-diff` supporting ranking genes

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* turn off pylab
* fix a bug for function `rev_seq`, reported in issue 10
* update documentation

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* ``brie-diff`` takes multiple cells, which could handle pair-wise comparisons
for 100 cells in ~10min with 30 CPUs; and 1000 cells within a few hours.
* Simulation wraps on Spanki are provided for simulating splicing events at
different coverages or drop-out probability and drop-out rate for single
cells: https://github.com/huangyh09/brie/tree/master/simulator

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* support Python 3.x now
* do not depend on h5py anymore for hdf5 data storage.
* ``brie-factor`` returns xxx.csv.gz rather than xxx.h5
* ``brie`` returns sample.csv.gz rather than sample.h5
* ``brie-diff`` takes sample.csv.gz rather than sample.h5