Fixed two py3k related bugs in pelican-quickstart

1. string in python3 doesn't have the decode() method. so when under py3k, pelican-quickstart should just use the string get with input().
2. if pelican is built with python3 but `python` is not linked to `python3`, `make serve` will complain "failed to import sitecustomize", because the makefile template still uses `python -m pelican.serve` to do the call, instead of `python3 ...`.

3. I don't understand why pelican-quickstart is using both _input_compat() and @decoding_strings. (So I just left a #FIXME comment there.)