Support for --platform flag to install command

I've documented the CLI help and the man page for bundle-install.

However, I need some help to automate the following test case if needed.

Here is what I ran manually:

```
$ mkdir bundler-tests
$ cd bundler-tests

$ cat <<EOF >Gemfile
source :rubygems 
gem 'jruby-jars', :platforms => :jruby
gem 'appengine-apis', :platforms => :jruby
EOF

$ bundle install --path .gems/bundler_gems --platform jruby 
Fetching source index for http://rubygems.org/ 
Installing jruby-jars (1.6.1) 
Installing jruby-rack (1.0.8) 
Installing rack (1.2.2) 
Installing appengine-rack (0.0.12) 
Installing appengine-apis (0.0.23) 
Using bundler (1.1.pre.4) 
Your bundle is complete! It was installed into ./.gems/bundler_gems 

$ rm -rf .bundle/ .gems/ Gemfile.lock 

$ bundle install --path .gems/bundler_gems 
Fetching source index for http://rubygems.org/ 
Using bundler (1.1.pre.4) 
Your bundle is complete! It was installed into ./.gems/bundler_gems
```

This thread for more information:
http://groups.google.com/group/appengine-jruby/browse_thread/thread/21b6f9e87935836b