Use product_type slug not sanitized term name
Make slug elements ucfirst for class name

For example, instead of `WC_Product_Variable_subscription` create the
class name `WC_Product_Variable_Subscription`.
Use slug instead of sanitzed title of term to determine product type

Props @thenbrent for:
https://github.com/woothemes/woocommerce/commit/b2a868301c622d03702f5828
5cb6d97972511953#commitcomment-2663540
Merge pull request #2496 from thenbrent/slug-not-sanitize

Use product_type slug not sanitized term name
conditional for WC_Email template base in abstract constructor
Merge branch 'master' of github.com:woothemes/woocommerce