Allow the default sound to be suppressed (via setting 'sound' => null) when constructing a message

Replaced `isset()` for `array_key_exists()` in the `BaseOptionedModel` to better detect when options were set
but were null. (`isset` considers an array key with a value of `null` to be unset).
Updated  to use  as well
Merge pull request #8 from phindmarsh/silencer

Allow the default sound to be suppressed (via setting 'sound' => null) when constructing a message
BaseOptionedModel and BaseParameteredModel adapted to PR #8
Some APNS adapter tests added
GCM adapter supports improvements + Some tests added
README updated
README updated
Merge branch 'master' of github.com:Ph3nol/NotificationPusher into dev
GCM data message bug fixed
README minor update
First Console elements
PushCommand update
Documentation updated with CLI push
PHPCS fixes
Composer versions updated
Added ability to set custom data in the APNS adapter
Added default parameter (empty array) for custom data in APNS
Merge pull request #11 from phindmarsh/custom-data

Added ability to set custom data in the APNS adapter
Some typo fixed