Added file URI handling for nodeDownloadRoot and npmDownloadRoot (issue 27)

Adds support for having a local cache of node and npm as asked on issue https://github.com/eirslett/frontend-maven-plugin/issues/27
basically you have to set nodeDownloadRoot to something like 'file:/your/path/to/node' without the v1.2.3 part and without the tar.gz or exe
E.g. to keep a cache for version 0.10.33 of node and version 1.4.24 of npm in a project sub directory named "cache" you can set:
```
<plugin>
  <groupId>com.github.eirslett</groupId>
    <artifactId>frontend-maven-plugin</artifactId>
    <version>...</version>
    <configuration>
      <nodeDownloadRoot>${project.baseUri}cache/</nodeDownloadRoot>
      <npmDownloadRoot>${project.baseUri}cache/</npmDownloadRoot>
      ...
    </confifuration>
</plugin>
```
The "cache" directory structure should be populated as following on Linux:
```
cache/
├── npm-1.4.24.tgz
└── v0.10.33
    └── node-v0.10.33-linux-x64.tar.gz
```

And on Windows (x64):
```
cache/
├── npm-1.4.24.tgz
└── v0.10.33
    └── x64
        └── node.exe
```