Robustness: wrap any calls to System.getProperty in case security is enabled.

Since calling System.getProperty may throw an exception on locked-down systems.

Note: this patch has the side-effect of making the 'guice_include_stack_traces'
setting static, rather than querying the system property each time a module is
installed. Making the setting static improves performance as System.getProperty
can be a bottleneck, at the cost of being able to change the setting on-the-fly.

If the 'guice_include_stack_traces' setting should be dynamic then I can amend
the patch accordingly.
@mcculls You could take in a default value and a "secure" value, like this:

```java
static String getProperty(final String name, final String defaultValue, String secureValue) {
  try {
    return AccessController.doPrivileged(new PrivilegedAction<String>() {
      @Override
      public String run() {
        return System.getProperty(name, defaultValue);
      }
    });
  } catch (SecurityException e) {
    return secureValue;
  }
}
```