Feature/issue 413 cholesky corr type

#### Summary:

This adds the proper lkj_corr distribution for Cholesky factors of correlation matrices.

#### Intended Effect:

Add the distribution.

#### How to Verify:

Run the tests.

#### Side Effects:

None.

#### Documentation:

It's in the manual.

#### Reviewer Suggestions:

Ben or Daniel should check this is tested thoroughly enough (I just got the merge done and made this request).