Improvements of "Api::getResolutions" method

[aik099]

The Api::getResolutions method (added in 2.0.0):

• have wrong DocBlock (the non-existing $project_key parameter)
• doesn't cache it's results in Api class as similar non-project/issue specific methods do (e.g. getStatuses and getPriorities)

[jpastoor]

Initially developed in PR #104