Make scripts in rlx_prv_assembler DTL templates

This moves scripts out of the rlx_prv_assembler and into their own DTL template files in priv/templates.

This requires the kaos:erlydtl branch to get merged into rebar. See [rebar PR#221](https://github.com/rebar/rebar/pull/221). Otherwise template compilation will fail with an error similar to the following:

        Compiling priv/templates/erl_script.dtl failed:
        DEBUG: Worker compilation failed: {{error,
                                            {'EXIT',
                                             {{case_clause,{ok,erl_script_dtl}},
                                              [{rebar_erlydtl_compiler,do_compile,4,
                                                [{file,
                                                  "src/rebar_erlydtl_compiler.erl"},
                                                 {line,208}]},
                                               {rebar_base_compiler,compile,3,
                                                [{file,"src/rebar_base_compiler.erl"},
                                                 {line,121}]},
                                               {rebar_base_compiler,compile_worker,3,
                                                [{file,"src/rebar_base_compiler.erl"},
                                                 {line,198}]}]}}},
                                           {source,"priv/templates/erl_script.dtl"}}