Color init with string: use Color resource management for setup

When initializing a Color by string, a PixelWand is leaked every time.

This can be easily verified by calling

```for i in xrange(5000):
    orange = wand.color.Color('orange')
    del orange```

and watching the memory footprint of the Python process increase (with any system tool available).

Fix: freeing the PixelWand in __init__. I thought it might be best to use the resource management that's already in place instead of adding the DestroyPixelWand explicitly.

No test is added, because I don't know about a cross-platform builtin Python module to measure memory.

All provided tests still pass.