curl: add option to use alternative *SSL providers, use curl-ca-bundle

add option to use alternative *SSL providers: gnutls or polarssl
suggest to use curl-ca-bundle to have working version of curl and libcurl

rationale:
osx default openssl library has osme bugs, one of those is support os SNI (SSL Virtual Hosts) which is broken - already exists option to choose brewed version of open ssl, but there are other providers like GnuTSL and PolarSSL that are working fine with curl.

other issue is that brewed curl, do not integrate with osx keychain, so there are no root certificates, this cripples any software that is using libcurl, as not all of them have support to specify directly where is a ca-bundle.