Replaced shorthand character classes matching digits in Regular Expressions

`\d` is a commonly used shorthand character class which can be found in many Regular Expressions and is the shorthand for `[0-9]`, however it is not supported in the [dk.brics.automaton](http://www.brics.dk/automaton/) package which is currently the underlying deterministic finite-state automata (DFA) engine in AutoFixture.

This Pull Request is related to this [work item](http://autofixture.codeplex.com/workitem/4254) as well as to #23. It replaces shorthand character classes matching digits so that AutoFixture can generate a string matching Regular Expressions e.g. `\d{8}`, `\d{5}(-\d{4})?`instead of returning a `NoSpecimen` request.