Add gemspec

I was trying to install this Gem straight from Github via Bundler and received the following error message:

```
Updating git://github.com/vmware/rbvmomi.git
Fetching source index from https://rubygems.org/
Resolving dependencies...
Could not find gem 'rbvmomi (>= 0) ruby' in git://github.com/vmware/rbvmomi.git (at master).
Source does not contain any versions of 'rbvmomi (>= 0) ruby'
```

It would just work fine by adding the gemspec to the repository.