Freeze on match customization

Addresses #120 by introducing a new customization that freezes specimens based on a set of criteria.

### Usage

The `FreezeOnMatchCustomization` uses the `IRequestSpecification` abstraction as a way to match the incoming requests with the frozen specimen. For example:

```csharp
var fixture = new Fixture();
var freeze = new FreezeOnMatchCustomization(
    typeof(string),
    new PropertySpecification(typeof(string), "Foo"));
fixture.Customize(freeze);
```

This will assign the same string value to _all properties of type `string` named `"Foo"`_.

Multiple matching criteria can also be combined using the `OrRequestSpecification`. Here's an example:

```csharp
var fixture = new Fixture();
var freeze = new FreezeOnMatchCustomization(
    typeof(string),
    new OrRequestSpecification(
        new PropertySpecification(typeof(string), "Foo")
        new ParameterSpecification(typeof(string), "bar"))); 
fixture.Customize(freeze);
```

When no matching criteria are specified, the `FreezeOnMatchCustomization` behaves like the current `FreezingCustomization`, that is it will only match by _exact type_. More concretely:

```csharp
var freeze = new FreezeOnMatchCustomization(typeof(string));
```

is the same as saying:

```csharp
var freeze = new FreezeOnMatchCustomization(typeof(string), new ExactTypeSpecification(typeof(string));
```

### Follow-ups

The are a few things I deliberately left out until after I gather your feedback:

- Since the `FreezeOnMatchCustomization` effectively supersedes the existing `FreezingCustomization`, I propose we obsolete it and rename the `FreezeOnMatchCustomization` to simply be `FreezeCustomization` for consistency.
- I'll add the XML documentation once the feature is finalized.

I look forward to your comments.
> What happens in nested inheritance hierarchies?

This specification works with _direct base types_ only.

In an earlier version of it, I actually walked the entire inheritence tree all the way to the root to check for type compatibility. However, that created the undesirable side-effect of replacing _everything_ with the frozen specimen since, ultimately, all types inherit from `object`.
I considered limiting it to check all types up to but _excluding_ `object` but that also broke the [principle of least surprise](http://en.wikipedia.org/wiki/Principle_of_least_astonishment). In the end, I decided to go for the direct base type because that was more conformant to what one would consider to be the expected behavior, IMO.

However, the specification is misnomed. I'd rather change it to be `DirectBaseTypeSpecification`.