Added Digest and Basic authentication adapters

This simplifies the API to just configure and authenticate.  The Basic authentication http adapter is working but the Digest is not.  I took a pass at adapting https://github.com/stormpath/stormpath-sdk-php/blob/master/Services/Stormpath/Http/Authc/Sauthc1Signer.php to the https://github.com/TomHAnderson/stormpath-sdk-php/blob/Digest/src/Stormpath/Http/Client/Adapter/Digest.php adapter but request are failing.

I added phpunit testing.  To setup your environment run
```
./composer.phar install --dev 
```
Then from the project root run ```phpunit```.  This will run two unit tests including one failing testing the Digest authentication.