Support for partial versions

This Pull Request addresses #14 by adding support for versions with less than four components.

The following cases are now supported:

```
AssemblyVersion("2.1")
AssemblyFileVersion("2.1")

AssemblyVersion("2.1.4")
AssemblyFileVersion("2.1.4")
```

Because [an ArgumentException is thrown when initializing a Version instance with fewer than two components in the constructor](http://msdn.microsoft.com/en-us/library/y0hf9t2e.aspx) the following cases are not supported:

 ```
AssemblyVersion("2")
AssemblyFileVersion("2")
```

The proposed implementation is based exclusively on Regular Expressions by turning minor, build, and revision components into [Optional Items](http://www.regular-expressions.info/optional.html) thus expanding the Capturing Groups of the pattern matching.