Added IsPropertyFalse/-True conditions

Added `<ispropertytrue property="foo"/>` and `<ispropertyfase property="foo"/>` condition.

```
        <property name="test" value="yes" />
        <if>
            <ispropertytrue property="test"/>
            <then>
                <echo message="foo"/>
            </then>
        </if>
        <if>
            <ispropertyfalse property="test"/>
            <then>
                <echo message="bar"/>
            </then>
        </if>
```