replaced hardcoded tool-locations with configurable tool-locations

Note that this was kind of an excercise for me, to get known to gradle concepts. Maybe my changes help you improve your plugin, maybe not:)

Main-Fix: replaced hardcoded tool-locations with configurable tool-locations (was on TODO-list of odi-um).

In order to make the tool-locations configurable, I added the project.extension "php", that enables the configuration of the relevant properties (and could be easily extended for more options (see README.md for new settings-options)).
The Tasks have been modified to use dynamic values instead of hardcoded ones.

NOTE: I have changed the commandline-parameters of phpunit in PhpUnitTask from ´phpunit -c ${dir}´ to ´phpunit ${dir}´ (could be reenabled, when a custom-configuration-file would be added to the configuration (PhpPluginExtension.groovy)).

Also changed the following:
* fixed build.gradle (wouldn't resolve junit-dependency on my machine)
* fixed META-INF/*.properties filename. AFAIK, for community-plugins, this should be the fully qualified package-name.

The latter changes the way, the plugin is applied to "apply plugin: 'com.eltrino.gradle.php'" which is reflected in the updated README.md