Feature/issue 942 reorg common

#### Summary:

Reorganized code currently in src/stan/common into src/stan/services (common code of which the interfaces can take advantage) and src/stan/interface (abstract virtual classes that the interfaces need to implement in order for those services to work).

#### Intended Effect:

One set closer to no code duplication in the interfaces.

#### How to Verify:

No user-facing effects.

#### Side Effects:

The interfaces will have to be updated as they all use some code that was in src/stan/common.

#### Documentation:

Not user-facing.

#### Reviewer Suggestions:

Anyone.