Metadata tags leak memory

A call to ImageMetadata.write() is not needed for the leak to be observed, I updated the test case accordingly.

There are in fact two types of leak that can be observed here:

1) The memory allocated when instantiating an {Exif,Iptc,Xmp}Tag without a value is not freed, it can be observed with the following piece of code:

    for i in xrange(1000000):
        tag = pyexiv2.ExifTag('Exif.Image.Artist')
        # - or -
        tag = pyexiv2.IptcTag('Iptc.Application2.Subject')
        # - or -
        tag = pyexiv2.XmpTag('Xmp.dc.subject')

2) When setting the values of an IptcTag, a copy is performed but the memory allocated is never freed, it can be observed with the following piece of code:

    values = ['bla'] * 10000
    for i in xrange(1000000):
        tag = pyexiv2.IptcTag('Iptc.Application2.Subject')
        tag.values = values

The first type of leak is already fixed in the branch linked.

After more tests, it appears that the second type is in fact not a leak. The very large size of the test values was to blame, and although the memory consumption went quite high (~130MB), it then remained stable. Testing with a smaller list of values (e.g. 100 elements, which is already quite large for real-world use cases) shows a small and constant memory footprint.