adds make install target

This commit adds an `install` target to `Makefile`. It is configurable by `prefix` (defaults to `/usr/local`) as well as `DESTDIR` (defaults to empty).

Thus:

-   install to `/usr/local`:

        make install

-   install to `/usr`:

        make prefix=/usr install

-   install to `/usr` with another destination directory:

        make prefix=/usr DESTDIR=$HOME/apps/bioinf install

The target depends on `all` and copies it to the `bin` directory within `prefix`.