Event Manager Improvements and Documentation

@lphuberdeau sorry for being so late with another PR... It contains improved events and some general simplifications including full-blown documentation ;)

## Doctrine-Style event namings
I have renamed existing events and added some new ones, to go as near as possible with the [Doctrine ORM Events](http://docs.doctrine-project.org/en/2.0.x/reference/events.html). The following events are available, working and tested:
* [prePersist](/lib/HireVoice/Neo4j/Event/PrePersist.php) - Fires before an entity is persisted
* [postPersist](/lib/HireVoice/Neo4j/Event/PostPersist.php) - Fires after an entity is persisted
* [preRelationCreate](/lib/HireVoice/Neo4j/Event/PreRelationCreate.php) - Fires before a relation is created
* [postRelationCreate](/lib/HireVoice/Neo4j/Event/PostRelationCreate.php) - Fires after a relation is created
* [preStmtExecute](/lib/HireVoice/Neo4j/Event/PreStmtExecute.php) - Fires before a statement (query) is executed
* [postStmtExecute](/lib/HireVoice/Neo4j/Event/PostStmtExecute.php) - Fires after a statement (query) is executed
* [preRemove](/lib/HireVoice/Neo4j/Event/PreRemove.php) - Fires before an entity is removed
* [postRemove](/lib/HireVoice/Neo4j/Event/PostRemove.php) - Fires after an entity is removed
* [preRelationRemove](/lib/HireVoice/Neo4j/Event/PreRelationRemove.php) - Fires before a relation is removed
* [postRelationRemove](/lib/HireVoice/Neo4j/Event/PostRelationRemove.php) - Fires after a relation is removed

Btw. I moved some tests out of the EntityManagerTest to the newly created [EventManagerTest](https://github.com/frne/Neo4j-PHP-OGM/blob/event-manager-documentation/lib/HireVoice/Neo4j/Tests/EventManagerTest.php) to keep it clean...

## Documentation
I started writing a documentation describing the usage of the doctrine events. IMHO, this would blow the readme.md and kill readability. Therefore, i created a [doc directory](https://github.com/frne/Neo4j-PHP-OGM/blob/event-manager-documentation/doc/index.md). There is some different documentation. I would find it nice, if we can present the readme.md as "overview at a glace" and keep it short. Therefore, I moved also some existing documentation to the doc directory...

Also added documentation for [events](https://github.com/frne/Neo4j-PHP-OGM/blob/event-manager-documentation/doc/events.md) and [annotations](https://github.com/frne/Neo4j-PHP-OGM/blob/event-manager-documentation/doc/annotations.md).

This is a follow-up for #58 and #65