freetds.rb: Enable support for building FreeTDS from HEAD (gitorious) + add test + enable --universal

```
marca@marca-mac:/usr/local/Library/Formula$ brew install freetds --HEAD --universal
==> Cloning https://git.gitorious.org/freetds/freetds.git
Updating /Library/Caches/Homebrew/freetds--git
==> autoreconf -i
==> ./configure --prefix=/usr/local/Cellar/freetds/HEAD --with-openssl=/usr/bin --with-tdsver=7.1 --mandir=/usr/local/Cellar/freetds/HEAD/share/man
==> make
==> make install
[] /usr/local/Cellar/freetds/HEAD: 63 files, 3.6M, built in 67 seconds
marca@marca-mac:/usr/local/Library/Formula$ brew test freetds --HEAD -v
Testing freetds
==> /usr/local/Cellar/freetds/HEAD/bin/tsql -C
/usr/local/Cellar/freetds/HEAD/bin/tsql -C
Compile-time settings (established with the "configure" script)
                            Version: freetds v0.92.dev.20130718
             freetds.conf directory: /usr/local/Cellar/freetds/HEAD/etc
     MS db-lib source compatibility: no
        Sybase binary compatibility: no
                      Thread safety: yes
                      iconv library: yes
                        TDS version: 7.1
                              iODBC: yes
                           unixodbc: no
              SSPI "trusted" logins: no
                           Kerberos: no
marca@marca-mac:/usr/local/Library/Formula$ file /usr/local/bin/tsql
/usr/local/bin/tsql: Mach-O universal binary with 2 architectures
/usr/local/bin/tsql (for architecture i386):	Mach-O executable i386
/usr/local/bin/tsql (for architecture x86_64):	Mach-O 64-bit executable x86_64
```