Do not convert keys to strings in Dash and Trash.

This addresses #151. It's a pretty significant API change, however for anyone who was accessing properties in Dash/Trash with a string key.

#### Before

``` ruby
require 'hashie'

class Person < Hashie::Dash
  property :name
end

p = Person.new(name: 'dB.')

p.name # => 'dB.'
p[:name] # => 'dB.'
p['name'] # => 'dB.'

# not what I put in
p.inspect # => { 'name' => 'dB.' }
p.to_hash # => { 'name' => 'dB.' }
```

#### After

```ruby
require 'hashie'

class Person < Hashie::Dash
  property :name
end

p = Person.new(name: 'dB.')

p.name # => 'dB.'
p[:name] # => 'dB.'
# p['name'] # => NoMethodError

p.inspect # => { :name => 'dB.' }
p.to_hash # => { :name => 'dB.' }
```