Update bitbucket_rest_api gem.

This will enable updating pull requests.
Create BuildLog class to parse and regenerate PR's descriptions.
Add initial implementation of pull request updater.
Allow a more concise markdown with image urls at the bottom of description.
Support links on markdown generation.

This code is messsy. I'll extract it to another class.
Extract build log line generation and parsing to a separate class.
Add spec for #pull_request method.
Pairing with Iuri
Fix tests and implement more description generation.
Merge pull request #2 from digitalnatives/update_pull_requests_descriptions

Update pull requests descriptions