Add typer target (rebase of #309)

This patch adds a typer target to the Makefile. It makes it
easier for developers to start adding specs to the codebase. It
also as a build_plt target with the built-in applications typer
needs to analyze rebar.

This also includes the Erlang version in the generated PLT file.
typer and dialyzer targets use this file. Finally, the distclean
target removed the versioned PLT file.