Adding a STATIC_DIR setting to control the output location of static content

This change adds a configuration variable STATIC_DIR (default 'static') that controls where the STATIC_PATHS are copied to.

I am currently rebuilding a web site using Pelican and we need to control where the static content goes to retain compatible URLS.  In my case, STATIC_DIR = '.'

I have followed the contributing guide lines.  Not all tests pass, but the same tests fail as at ee752d9b, where I started.

I hope this is helpful.