ping server to keep connection open
Fix popup refreshing on send
Empty webcrypto spec impl

First steps toward #5.
Add CryptoJS impl of hmac-sha256 and aes-ctr

Just a holdover til Web Crypto gets here, I Promise.
Implement TextSecure HKDF in terms of web crypto

We won't be able to use web crypto's HKDF implementation since ours is
sligtly non-standard, but we can use their HMAC-SHA256.
Rewrite verifyMACWithVersionByte in terms of web crypto
Rewrite calculateMACWithVersionByte in terms of web crypto
Rewrite FillMessageKeys in terms of web crypto
kill unused hmac fn