Azure support 'w' mode on open

[guydou]

Currently in the azure backed I can't do the following:
f= default_storage.open("bla", "w")

if the blob bla doesn't exist

and also I can not update bla with new content, if I use: default_storage.save("bla", content) and bla already exists it will a new blob will be created with a random suffix

for example:
bla
bla_MhgTcjK

[guydou]

this pull request fixes the problem #318

[sww314]

Azure backend was rewritten in #565. Please reopen if the issue still exists.