Update JWE.php

Modified decrypt method to accept callback as $private_key_or_secret parameter's value.
Update JWE_Test.php

Added test for encryption key as callback
Update README.md

Changed build status link
Update README.md

Updated link to build icon
Update JWE.php

Fixed undefined index error.
Update JWE_Test.php

Fixed pass $this in scope of anonymous function
Update JWE_Test.php

Fixed test
Finally fixed decryption of JWE using callback as key
Merge pull request #6 from peter-svintsitskyi/master

Update JWE.php