Support latest JIRA REST API and add more methods into the implementation.

[ijab]

• Modified to support v2 JIRA REST API(keep supporting v1 for backward compatibility)
• Let api_version & api_prefix be configurable
• Add some more methods into the implementation(Some methods using payload need Net::HTTP::Spore v0.06 for v0.03 can't send palyload correctly.)
• Changed the return value of each method to the body if status is 2xx for matching the manual of JIRA REST APIs and convenience while using