Implement strptime

Hi Alon,

I implemented the missing strptime function, including tests. Please note that "alternate locale representation" support (%E..., %O...) support is currently missing as well as GNU extensions (%x, %s). Other than that, the specification from http://pubs.opengroup.org/onlinepubs/009695399/functions/strptime.html is implemented. 

If you are happy with my changes, may I be so bold to ask for my inclusion into AUTHORS? (Soeren Balko - Soeren.Balko@gmai.com)

Cheers,
Soeren