Missing deps

Implements feature to be used in builder as discussed
@movitto 
Do we have easy access to the missing dependency requirements after calling this method?

In other words, if we ask koji for bundler ~>1.6.2 and rspec ~> 3.0 and get this logging:
```
D, [2014-07-15T12:20:04.803952 #9271] DEBUG -- : versions_for<koji>(bundler)...
D, [2014-07-15T12:20:05.147649 #9271] DEBUG -- : ["1.5.2", "1.3.5", "1.3.1", "1.1.4"]
D, [2014-07-15T12:20:05.707613 #9271] DEBUG -- : versions_for<koji>(rspec)...
D, [2014-07-15T12:20:06.000202 #9271] DEBUG -- : ["2.14.1", "2.13.0", "2.8.0"]
```

We get that bundler and rspec are missing but lose the fact what version constraint we wanted and what gems version were found but failed to meet the constraint.

I believe a client caller would want to know what gems are missing, what constraints were checked and what was available as an alternative.

Perhaps we can return either an associative array or an object? 
For example: 
```ruby
 [["bundler", "~>1.6.2"], ["rspec", "~>3.0"]]
```
Or perhaps something like this:
```ruby
[
MissingDependency @dependency=Gem::Dependency..."bundler", "~>1.6.2", @found = {:koji => ["1.5.2", "1.3.5", "1.3.1", "1.1.4"]}
MissingDependency @dependency=Gem::Dependency..."rspec", "~>3.0", @found = {:koji => ["2.14.1", "2.13.0", "2.8.0"]}
]
```