Grunt tasks fail on Windows

[mhofman]

There are some small linux dependencies in the Gruntfile.js files:

• In src/Gruntfile.js, the task to copy the runtime jar to dist uses the cp executable.
• In Gruntfile.js, the runMetadataGenerator exec task is not invoked in a cross platform way. Currently the generator is also not cross platfrom itself, pending a fix for Bash dependency is not Windows friendly android-metadata-generator#7

I have a pull request incoming for this.

[blagoev]

@mhofman This has been merged into master. Thanks for the fix.