Update crypto, token expiration, ECDH fix

Updated the crypto module to v0.9.0
ECDH output on P-256 curve must be always 32 bytes .
Added the ability to specify Vapid JWT token expiration

SendNotificationWithContext

Added a SendNotificationWithContext function that accepts a context for supporting telemetry solutions.

JWT Upgrade

Replaced the JWT library with golang-jwt/jwt.

Fixes #42

Return error when exceeding maximum payload size

Fixes #27

Check base64 encoding

Encodes and decodes VAPID base64 keys to ensure they are in the correct format.

Fixes #26
Fixes #29

Optional Record Size

Added RecordSize to the Options struct. Limits the record size of the encrypted payload.

HTTP Client Interface

Re-exporting the HTTP Client Interface to maintain compatibility with v0.0.1.

aes128gcm encoding

List of changes described in: #17

aesgcm encoding

v0.0.1

Fix import path in example. (#15)