Add JiraAgent

The agent does periodical queries and emits
the events containing the updated issues in JSON format.

The agent also supports HTTP basic authentication.
Fix date/time manipulations in jira_agent.rb

During testing, I figured out that jira doesn't understand
timezones in JQL. Because of this I have to introduce a
workaround that fetches issues 24 h earlier that the requested
date and then filters out unnecessary ones based on the
value of 'updated' field.
Addressed review comments by @cantino
fix typo in jira_agent.rb
introduce request guards in jira agent
rename MAX_REQUESTS to MAX_EMPTY_REQUESTS in jira agent
make JiraAgent more testable and fix some minor issues detected during testing
add RSpec tests for JiraAgent
Merge pull request #313 from racktear/jira_agent

Add JiraAgent