Set each gemfile to separate directory

Set ```path/to/Gemfile```, then automatically create ```path/to/.bundle/config```, so both ```path/to/foo.gemfile``` and ```path/to/bar.gemfile``` override same file, ```path/to/.bundle/config``` .

This change makes install easily:
```
$ bundle --gemfile=gemfiles/rails3.2/Gemfile --path=.bundle
# this install gems to gemfiles/rails3.2/.bundle/xxx
$ bundle exec ruby test/test_foo.rb BUNDLE_GEMFILE=gemfiles/rails3.2/Gemfile
```