Add getAllowFlight() and setAllowFlight(boolean) methods.

<h3><i><b>In this pull request I have:</b></i></h3>
<ul>
	<li>Added a <code><a href="https://github.com/lexware/SpongeAPI/blob/458394a2757eb98a72b9013905431e84e105c460/src/main/java/org/spongepowered/api/entity/Player.java#L66">getAllowFlight()</a></code> method in the <a href="https://github.com/lexware/SpongeAPI/blob/master/src/main/java/org/spongepowered/api/entity/Player.java">Player.java</a> interface.</li>
	<li>Added a <code><a href="https://github.com/lexware/SpongeAPI/blob/458394a2757eb98a72b9013905431e84e105c460/src/main/java/org/spongepowered/api/entity/Player.java#L73">setAllowFlight(boolean)</a></code> method in the <a href="https://github.com/lexware/SpongeAPI/blob/master/src/main/java/org/spongepowered/api/entity/Player.java">Player.java</a> interface.</li>
</ul>