Add support for detecting an existing phantomjs installation.

Add option that skips downloading if phantomjs is already present on the systems PATH.

Here's an example:

```xml
<configuration>
  <version>1.9.2</version>
  <checkSystemPath>true</checkSystemPath> <!-- default is false -->
</configuration>
```

Of course, just because phantomjs is on the PATH doesn't mean it is the correct version.  So we will need to make the plugin check that it is the correct version.  However, I'm sure there will be cases where users would rather just accept any version so we will need a parameter to control this behavior:

```xml
<configuration>
  <version>1.9.2</version>
  <checkSystemPath>true</checkSystemPath> <!-- default is false -->
  <enforceVersion>false</enforceVersion> <!-- default is true, only applies when checkSystemPath is true -->
</configuration>
```

We may want to even extend the `enforceVersion` parameter to allow a syntax similar to maven's for [specifying a version range](http://books.sonatype.com/mvnref-book/reference/pom-relationships-sect-project-dependencies.html#pom-relationships-sect-version-ranges).  Then we could enforce that the version just fall within that range.

For instance, this says to check the system path for phantomjs and if it is found and the version is greater than 1.9.0 than use that version, otherwise, download version 1.9.2:

```xml
<configuration>
  <version>1.9.2</version>
  <checkSystemPath>true</checkSystemPath> <!-- default is false -->
  <enforceVersion>[1.9.0,]</enforceVersion> <!-- default is ${phantomjs.version}, only applies when checkSystemPath is true -->
</configuration>
```