Assume build command by default and default platform

Most of the time, all people want to do is build. Plus; it feels very counter-intuitive to execute `phonegap-build build` all the time. Moreover, most of the time we will want to build for the same platform.

This pull request:
* Makes the command optional, defaulting to the `build` command if none is specified.
* Makes the platform optional, defaulting to `android` when none is specified.
* Allows *building* for multiple platforms (however, the current API does not allow for build callbacks for multiple builds, hence I left it undocumented).

This should pave the way for:
* A multi-platform build, to be implemented later, together with;
* Per project ('local') platform defaults, to be prompted for during the `create` command and stored in `.cordova/config.json` within the project.

For now this code will be used by us internally. As a merge would imply certain non-trivial design decisions (i.e. defaulting to Android) and we are open to feedback on suggestions for improvements. Meanwhile, we think it's important to 'have the code out there' for other people to use. Hence the pull request. :)