adding bounds tests for dirichlet; currently fails
dirichlet: adding test for mismatched sizes
dirichlet: checking mismatched sizes
dirichlet: checking for alpha > 0
dirichlet: checking theta is a simplex
dirichlet: adding bounds tests for vars
Merge pull request #254 from stan-dev/feature/issue-247

Fixes #247.
Merge branch 'develop' into feature/issue-259