generified bson macros type signatures

generated handlers matched some other BSON-related types but compiler
wouldn't agree due to sub-typing hierarchy. It needed some convincing
in terms of explicit type annotations.
macros: fix for handling Seq[A] in case class parameters

Provide explicit (String, BSONValue) type annotation for every element
when constructing BSONDocument. Without this type checking failed with
weird existential type error. Error message had no real errors in it.
Apparently compiler just needed a little hint.
Merge pull request #56 from edofic/macros

updated macros.