Implement force budget option and acyclic SparseMAP implementation

[andre-martins]

• The force_budget option allows forcing a budget factor to have exactly K variables on, as opposed to <= K.
• The acyclic SparseMAP implementation avoids having to use the iterative algorithm for LP-SparseMAP when the factor graph is acyclic (e.g. a single factor). This should speed things up considerable. There is a flag autodetect_acyclic which can be turned off.