Allow mixing in multiple Virtus.modules

Known caveat: if one module wants constructors/coercions/mass-assignment
but the other one doesn't, the one that *does* want those features will
win since it mixes in the module.

fixes #260
fixes #274
Merge pull request #276 from trptcolin/multiple_modules

Allow mixing in multiple Virtus.modules

fixes #260
fixes #274