Feature/negate op in environment expressions

[barspi]

(this is equivalent to PR #518 done against current master)

Add support for environment expressions with a negate/NOT operator, such as ${!server.enabled:false}

Extensive test cases have been added.

Some observations:

• The operator only makes sense for "booleanish" values. That means, the property is resolved to true/false/yes/no, or their string equivalents (upper/lowercase)).
• All booleanish values, when negated are resolved down into "true"/"false" or true/false (depending on the retrieval through Environment.get() or SimpleConfiguration#getBoolean())
• When using SimpleConfiguration#getBoolean() an undefined property always resolves as false (original behavior).
  Therefore, when negating an undefined property, getBoolean() will return true
• When an undefined property is used in an expression with a default, the default is always honored (i.e. a getBoolean() on ${!__undefined__:true} will return true, and if calling Environment.get() directly, it will return the string "true")
• A negate operator will never negate the default value. It's assumed that the intended meaning of a default is to be used literally (when the property in question couldn't be resolved)