Use purescript-simple-dom for #4

This removes the entire DOM module and replaces it with equivalents in `purescript-simple-dom`.  Most of the changes were in Http to use `Data.DOM.Simple.Ajax`, which now contains a lot of the code that was here.  Exported types remain almost identical, with the exception of the `MozChunked` types (which don't actually exist -- they're just represented by the same underlying types in the response, but with different timing semantics).  This addresses #4.