Add a vips_version function

[chregu]

Returns the version number of the vips library

As mentioned in #14

[jcupitt]

Nice!

[jcupitt]

I added a note to the changelog and bumped the version to 1.0.8.

I won't upload to pecl for a few days in case some more changes become necessary.

I removed the strdup() -- I think this is not necessary with php7 since everything is a zend_string now.

There should probably be a matching change to expose this in php-vips somehow.