making possible to override environment variables by system properties

When we are deploying in an environment like Amazon, we may need to update an environment variable on demand. Nowadays, we need to change the `environment.properties` in order to make this happen.

This pull request, allow us to override this properties using system properties.

We only allow user's to override properties that exists in `environment.properties` and do not allow to define only in system property.

```
#environment.properties
my.prop hello
```
```
//System.setProperty("my.prop", "bye");
environment.get("my.prop"); //bye
//System.setProperty("not.in.properties", "foo")
environment.get("not.in.properties"); //throw NoSuchElementException
```
Thanks @csokol. I'll do this!

On Tuesday, November 11, 2014, csokol <notifications@github.com> wrote:

> In vraptor-core/pom.xml:
>
> > @@ -63,6 +63,12 @@
> >  			<artifactId>xstream</artifactId>
> >  			<version>1.4.7</version>
> >  		</dependency>
> > +		
>
> We can use guava's Strings class:
> http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Strings.html#isNullOrEmpty(java.lang.String)
> … <#1499fe486c772a65_>
> On Tue Nov 11 2014 at 3:23:23 PM Nykolas Laurentino de Lima <
> notifications@github.com
> <javascript:_e(%7B%7D,'cvml','notifications@github.com');>> wrote: In
> vraptor-core/pom.xml: > @@ -63,6 +63,12 @@ >
> <artifactId>xstream</artifactId> > <version>1.4.7</version> > </dependency>
> > + Should I create a ValidationUtil class to do this? I believe that
> notBlank method is something that we can use in many parts of the project.
> — Reply to this email directly or view it on GitHub <
> https://github.com/caelum/vraptor4/pull/875/files#r20164752>.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/caelum/vraptor4/pull/875/files#r20164940>.
>