Small bug fix + small feature

[jeroenvdgulik]

• added a bugfix. If rename() fails, installer will try manully copying/removing
• autoguess SPARKPATH in MY_Loader for the current dir, and parent dir (in case index.php is not in the root but in a subdir like public_html or htdocs)

[janeklb]

This would fix http://getsparks.lighthouseapp.com/projects/70187-getsparks-repository/tickets/31-problems-installing-sparks-undefined-installed_path