Cleanup: Increase code quality.

As PHP-VCR gets used more, there is a need for a very stable code which can be easily extended in the future. To ensure good code quality this cleanup will:

 - [x] Add PHP docblocks
 - [x] Code coverage > 90%
 - [x] Move Filters into own namespace
 - [x] Move Client, SoapClient and Assertion into Util namespace
 - [x] Try to not hit the network in unit tests (if necessary use a `@group uses_internet`)

Fixes #38.