Support zero-length sequences

[prihoda]

Add support for zero-length sequences.

Example FASTA file:

>A
ABC
>B
>C

>D
DEF
GH

Result: ['ABC', '', '', 'DEFGH']

Also enables slicing sequences to zero length, which corresponds to slicing python lists:

from pyfaidx import Fasta
a = Fasta('example.fa')[0]
print(a[0:])
# >A:1-3
# ABC
print(a[100:]
#>A:101-101
#

Please review all the changes, I tested on an example dataset but I cannot be sure that this will always work.

[prihoda]

Fixes #93

[mdshw5]

I'll take a look today. On first glance this makes sense, and I'll add at least one test case before merging.

[mdshw5]

It looks like this change not only fixes #93, but it doesn't break existing tests either. Great job @prihoda!. I'll merge and tag a new version (0.5.6) which will push to PyPI within the next hour.