verbose: Fix response parsing of empty lists

When GitHub replies with an empty json list "[]" this was wrongly
converted to a "None" type in Python.
verbose: Fix verbose printing of request body
verbose: Pretty-print parsed GitHub objects
verbose: Remove printing of json for issues

We already have the GitHub object printed, which contain the same
information.
issue show: Fix URL construction for comments
issue show: Fix printing when issues have no labels
issue show: Remove unused --full option
pull show: Show both regular and code comments

Fixes #83 and also retrieve labels for pull requests.
issue show: Improve issue formatting
issue show: Add --summary option

With this option, the issues are printed with the same format as the
`issue list` command.
issue show: Retrieve comments only if needed
pull show: Print review comments correctly

Review comments belonging to the same piece of code is now printed in
a group, leaded by the diff hunk that's being commented. A group of
review comments and regular issue comments are now separated by a more
graphical horizontal bar.