Feature/sum recorder

#### Summary:

This adds a new recorder for use in RStan. This does not affect any of current Stan's implementation.

#### Intended Effect:

RStan needs the sum of parameters. This will make it much easier to implement this functionality in RStan.

#### How to Verify:

New unit test in src/test/unit/common/recorder/sum_recorder_test.cpp. This is not used in Stan directly.

#### Side Effects:

No side effects.

#### Documentation:

Not user facing.

#### Reviewer Suggestions:

Anyone.