Store as front page

Closes #10.
Form button titles, icons and HTML output are now filterable
Fix incorrect thumbnail size for no product image
More reliable way to regenerate thumbnail sizes lazily when size options change.
Allow more flexible sizes for product thumbnail.
Fix single thumbnail width in _wpsc_get_inline_style()
Fix: if "index permalink scheme" is being used, store rewrite rules are messed
up.

Fixes #8.
Modify @since tag for documentation