added support for bhkRagdollTemplate (.rdt files)

Added support for bhkRagdollTemplate block based on two .rdt files (.rdt file is a .nif file) located in Fallout3 directory "meshes/ragdollconstraint". It seems that game uses only file "Default.rdt". Second file "stiff.rdt" is unused and suspicious because in NiHeader it has User Version 2 set to 16 instead of 34 as all Fallout3 files (and also Default.rdt) have.
Can anybody check Fallout New Vegas files if there are present any .rdt files? It would be good to have more than 2 files for decoding. Or exists other games with these files?
Merge pull request #22 from ttl269/feature/bhkRagdollTemplate-support

bhkRagdollTemplate support
bhkBreakableConstraint update

Added structure of bhkBreakableConstraint. It seems that it is (and also bhkMalleableConstraint) some kind of "parent descriptor" for behavior for their "children" constraint which must be inserted within it. This inserted constraint in named "Sub Constraint", its type is determined by "Constraint Type" integer value. After this integer there is inserted complete constraint - one of allowed "common" constraints. And then there are inserted parameters specific for parent constrains (for example breakable one has "Threshold" and "Remove if Broken"). From vanilla nifs it seems that this "Sub Constraint" has always set its "Num Entities" to 2 but also both references to "Entities" are set to None. I think that this is probably because the relation between two rigid bodies this constraint is affecting is determined earlier - in "Entities" of parent constraint (bhkBreakableConstraint or bhkMalleableConstraint) and therefore it is not necessary (or it is forbidden?) to set references to entities also in "Sub Constraint".
added PrismaticConstraint to SubConstraints

1. Added PrismaticConstraint to list of available constraints in SubConstraint
2. Previously added enum "ConstraintType" renamed to "hkConstraintType"