Some fixes and extra support

[jeremy-compostella]

These patches addresses:

1. A systematic crash in `ox-jira-src-block' with the C source block with captions
2. The missing the verse block support
3. The missing entity support
4. The missing line break support

[stig]

Hello! Thank you for contributing!

I want to start by apologising for taking so long to get back to you on this PR. I appear to have accidentally unsubscribed from this repo at some point. I'm now watching it, so should be able to keep up with requests.

Regarding the PR itself:

1. The file you have touched is actually generated from the real source, found in the ox-jira.org file. That is an literate programming document, which is tangled to the ox-jira.el. Would you be open to add your changes to the ox-jira.org file? If it proves troublesome, I'm happy to help.

2. Are you able to add token tests for the changes you're making? There's no literate document for the tests, so that should be a bit easier.