Break apart the crypto methods in storage.connection for signing strings with a p12 key

[jgeewax]

This likely should live inside gcloud/credentials.py but is too big of a task for the "ability to sign URLs" task (#56)

CC: @proppy

[silvolu]

@jgeewax why did you want this into gcloud/credentials.py? Are there other use cases not Cloud Storage related for this?

[jgeewax]

I believe this was @proppy 's idea... can't recall the specific context. If it doesn't make sense, feel free to chop.