Add SW information to techatnyu.org

1. Add Startup Week poster as background splash image when first landing on the site. @abhinayashutosh 
2. Add announcements box with link to nyusw.com site, similar to lexilewtan.com @freialobo 

@ethanresnick @grungerabbit @abhiagar can one of you try these changes on your computer? Look for image problems with the startup week poster and for how to make the announcements box look better.