Add importKey to webcrypto.js
Finish up webcrypto integration, Fixes #72

We now correctly and opportunistically use the webcrypto API if
available, polyfilling if it's not detected. This change also includes a
layer of abstraction over the webcrypto interface so we no longer have
to deal with key-imports or algorithm names all over the place. Since we
no longer support AES-CTR, code outside this file can simply call
`textsecure.subtle.<encrypt|decrypt|sign>(key, data [, iv])`.