Python 2 fixes + insert size distribution

[HadrienG]

This PR addresses the bugs from #45 (and more):

Python2 related:

• zip(*args.ncbi, *args.n_genomes) not supported. Changed syntax.
• changed import httpto import http.client to catch the incomplete download/N exceptions
• insertions are str and not unicode anymore

Not python2 related:

• iss does not skip a genome if its length is shorter than the random insert size drawn from the model. Produces a random insert size within acceptable limits instead.

[codecov]

Codecov Report

Merging #46 into master will increase coverage by 0.71%.
The diff coverage is 87.87%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   73.97%   74.69%   +0.71%     
==========================================
  Files          19       19              
  Lines        1122     1142      +20     
==========================================
+ Hits          830      853      +23     
+ Misses        292      289       -3

Impacted Files	Coverage Δ
iss/app.py	0% <0%> (ø)	⬆️
iss/download.py	0% <0%> (ø)	⬆️
iss/test/test_generator.py	100% <100%> (ø)	⬆️
iss/generator.py	92.79% <100%> (+5.05%)	⬆️
iss/error_models/__init__.py	79.76% <100%> (ø)	⬆️

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