Allow creation of nitrate cases when exporting

[hegerj]

Added option --create which will create testcase in nitrate if it
doesn't exist.
Implements #117.

[hegerj]

@psss There are couple of TODOs left in the code, could you please check those?

[hegerj]

Because of the second function that needed nitrate I moved its import outside which brought back the import problem (thats why the checks are failing I believe).
Question is what to do about it.

1. move the import back into function export_to_nitrate() and import nitrate in create_nitrate_case() as well
2. move the import back into function export_to_nitrate() and pass the nitrate as parameter to create_nitrate_case()
3. rework create_nitrate_case() to not need nitrate and handle test case creation in export_to_nitrate()

What do you think/do you have different solution? Neither of those I proposed look good to me.

[psss]

I would suggest to import in both functions. Python does the import only once so it basically does not matter.

[psss]

Thanks for writing support for test case creation.
Adjusted in 3f8c604 and going to merge if tests pass.

[lgtm-com]

This pull request introduces 1 alert when merging 3f8c604 into 445751f - view on LGTM.com

new alerts:

• 1 for Module is imported with 'import' and 'import from'