Use isql-vt if available (e.g. Ubuntu)

When installing Virtuoso via APT on Ubuntu, the Virtuoso-specific SQL
interface is called `isql-vt`, in order to avoid clashes with unix-odbc.

See:http://virtuoso.openlinksw.com/dataspace/doc/dav/wiki/Main/VOSMake#The
isql command

To make the install script compatible, it now first checks for the
existence of `isql-vt` before falling back to `isql`.
Set default RAWBASE_HOME to the project root

Now, `RAWBASE_HOME` is set to the project root directory by default
(unless otherwise specified through the `-h` parameter).
Merge pull request #3 from pmeinhardt/improved-install

Improved install