Feature/issue 580 pareto type 2 distr

#### Summary:
Add the Pareto Type 2 distribution to stan (issue #580 ).

#### Intended Effect:
Add the Pareto Type 2 distribution to stan.

#### How to Verify:
Check function signature tests and distribution tests.

#### Side Effects:
None.

#### Documentation:
Added new distribution to manual.

#### Reviewer Suggestions:
Anyone.