Re-add distance functions to Stan

This resolves issue #283.

#### Summary:

Re-adds `distance` and `squared_distance` functions to the modeling language.

#### Intended Effect:

Added `distance` and `squared_distance` functions to math and agrad. Added these functions to the modeling language.

#### How to Verify:

Unit tests have been added:
- src/test/unit/math/matrix/distance_test.cpp
- src/test/unit/math/matrix/squared_distance_test.cpp
- src/test/unit-agrad-rev/matrix/distance_test.cpp
- src/test/unit-agrad-rev/matrix/squared_distance_test.cpp

Function signature tests has been added to:
- src/test/test-models/syntax-only/function_signatures_distance.stan
- src/test/test-models/syntax-only/function_signatures_squared_distance.stan
These can be run using target: `test/integration/compile_models`

#### Side Effects:

No.

#### Documentation:

Manual updated with distance and squared distance functions.

#### Reviewer Suggestions:

Daniel (I've already looked at it)