Improve variable substitution support

Variable substitution supports replacing strings of the form ${xxx} in the server side AMD config javascript and some servlet init params with values defined in Java system properties or by services that implement com.ibm.jaggr.core.IVariableResolver.  This change set extends the support to include:
* Variable substitution is now supported in all servlet and extension init-params.
* The global scope getProperty() function can be called by JavaScript code in the server-side AMD config as an alternative to using variable substitution.  For example: 
<pre>var foo = '${foo}';</pre>
assigns the same value to the variable foo as 
<pre>var foo = getProperty('foo');</pre>
if the property foo is defined.  If the property foo is not defined, then the var foo will equal '${foo}' in the first case and null in the second case.