Context: check required constructor parameters aren't null

[jnidzwetzki]

This PR introduces a null parameter check in the constructor. Especially when the ContextBuilder is used, it easy to forget required parameters (e.g., application) and the resulting Exception does not point out the missing parameter. With this patch, a customized NullPointerException is thrown.

== Old ==

Exception in thread "main" java.lang.NullPointerException
	at com.oanda.v20.Context.<init>(Context.java:93)
	at com.oanda.v20.ContextBuilder.build(ContextBuilder.java:100)

== New ==

Exception in thread "main" java.lang.NullPointerException : application can not be null
	at com.oanda.v20.Context.<init>(Context.java:92)
	at com.oanda.v20.ContextBuilder.build(ContextBuilder.java:100)