expose an issue with blosc_args handling of numpy arrays

When passing a BloscArgs object to pack_ndarray, the typesize isn't correctly
injected.
fix issue with typesize handling for numpy arrays
optimize logging in the inner loop during compression too
discuss and demonstrate inner loop debug optimization
v0.7.1 changelog
v0.7.1
Merge branch 'maint-0.7.x'

* maint-0.7.x:
  v0.7.1
  v0.7.1 changelog
activate python3 tests via travis
fix all relative imports to be py3 compatible
fix StringIO import to be py3 compat
iteritems missing in py3
use seek(0) instead of reset()

The BytesIO class seems not to have a reset().
fix use of xrange, now using six
make the magic a byte immediately
make the empty string and empty byte
indexing into a byte in python3 does a struct unpack, wtf?
strip bytes, not strings
encode the magic format and user_codec
zlib compress and decompress accept only bytes, not strings
use wb for good measure, not sure if required