Split "optional" dependencies into "recommended" and "optional".

curl is used quite a few times and is obviously recommended to have
intl is probably used for translation stuff and thus good to have
fileinfo is recommended as per https://github.com/owncloud/core/issues/5516#issuecomment-27016855

not sure about:
bz2
mcrypt
openssl (this is probably required for HTTPS support, though)

@danimo @DeepDiver1975 @karlitschek @PVince81