would be nice if we can have different code generators.
if i find the time i might implement that in the next days.

the idea of a *very* simple pluggable api:
* existing code generation goes into a separeate function
* code generation is announced by hook_commerce_coupon_code_generator_info() which gives a callback
* code generator is chosen globally (for now)
* commerce_coupon_generate_coupon_code() calls this callback with $codelength as argument

comments?