Bug fixes and Python 3 compatiblity

[saketkc]

Hi @rajanil,

This pull request:

• Makes the code Python 3 compatible without ensuring Python 2 compatibility
• Fixes load_data to skip transcripts with missing transcript_id as some features in
  newer versions of Gencode's GTF do not have it
• Some minor formatting changes to improve readability.

Here's a conda environment file: https://gist.github.com/saketkc/8af23c1bbe5c705885f1d28498c4e5e6
that might be useful for addressing installation issues reported #3 and #1.

Thanks for developing riboHMM!