Zend\Db support

Add support for Zend\Db for those trying to modernise their ZF1 deployments.

The plugin must be added with registerPlugin() and takes two parameters: adapter and backtrace. Adapter is an instance of Zend\Db\Adapter\AdapterInterface and backtrace is a boolean whether to include the call stack to the query.

This release also removes the deprecated Constants plugin (superseded by Variables) and cleans up the code a bit.

Improved timing

Update timing calculations to use more precise $_SERVER['REQUEST_TIME_FLOAT'] available since PHP 5.4

Allow Database plugin to trace queries

Extending Zend_Db_Profiler to generate a backtrace. This makes it easy to see, what method is responsible for each query.

1.6.3: Merge pull request #25 from brownoxford/add-composer-include-path

Add library directory to include path in composer