[C+B] Add api to use 1.7 chat features. Adds BUKKIT-5245

##### The Issue:
There is currently no easy way to use the new chat features introduced in 1.7.

##### Justification for this PR:
As said before, there is currently no way (except from directly using the ``/tellraw`` command and passing your own generated json to it) to use the 1.7 chat features.

##### PR Breakdown:
This PR adds a builder interface based on a ``RichMessage`` to which you append ``RichMessagePart``s to build a rich chat message containing (or not):
* a multi-color text, an LocalizedText, an ``ItemStack`` name or an ``Achievement`` name ;
* an optional hoverable (showing a multi-color multi-line text, an ``ItemStack`` or an ``Achievement``) ;
* an optional clickable (executing commands / sending chat message, proposing commands / proposing chat message, or opening urls (actually opens the prompt)).

These messages handle colors and formatting using ChatColor just like standard messages, internally using the existing algorithm built by the great @mbax.

Example LocalizedText:
```java
new LocalizedText("commands.scoreboard.players.reset.success", "Notch")
```

Many (**many**) shortcuts have been added to RichMessage to make it easier to append specific elements, see RichMessage constructors, append methods and insert methods.

``RichMessage`` and ``RichMessagePart`` are ``ConfigurationSerializable`` making it easy to save and load them from a config file.

This PR also adds methods to ``Player`` to send a ``RichMessage`` and two methods to ``Server`` and ``Bukkit`` to broadcast a ``RichMessage``. Those methods are mapped on the standards String methods.

Usage examples can be found [in the TestPlugin after this line](https://github.com/Ribesg/TestPlugin/blob/BUKKIT-5245/src/main/java/fr/ribesg/bukkit/testplugin/TestCommandExecutor.java#L63). You can get builds of all of this and test it yourself using the link below.

##### Testing Results and Materials:
[Test Plugin Source](https://github.com/Ribesg/TestPlugin/tree/BUKKIT-5245)
[Bukkit, CraftBukkit and TestPlugin builds on Jenkins](http://ci.ribesg.fr/view/Bukkit-PRs/job/PR-BUKKIT-5245/)

##### Relevant PR(s):
CB-#### - https://github.com/Bukkit/CraftBukkit/pull/#### - Associated CraftBukkit PR

##### JIRA Ticket:
BUKKIT-5245 - https://bukkit.atlassian.net/browse/BUKKIT-5245