Update README -- Remove SSL and reference tincan

Since the README was last updated, we removed all the SSL code from
ipop-tap, and moved it to Tincan. Additionally, building ipop-tap by
itself doesn't make much sense anymore for the end user, so I added
links to the wiki for building with ipop-tincan.
Merge pull request #15 from ipop-project/update-readme

Update README -- Remove SSL and reference tincan