Allow wildcards to be used when comparing a MIMEAccept against an offer

[ltvolks]

Primary motivation is to address Pylons/pyramid#1407, which would enable usage of wildcards as an accept predicate:

@view_config(accept='text/*')
def view(request):
    ...

Webob.acceptparse.MIMEAccept now supports the following, where previously an exception was raised:

>>> accept = MIMEAccept('text/html')
>>> 'text/*' in accept
True
>>> '*/html' in accept
True
>>> '*' in accept
True

• Fixes Accept matching against wildcard offers is not allowed #155
• Addresses Accept predicate with wildcard not allowed pyramid#1407

[digitalresistor]

Would you be so kind as to add some extra tests that would add coverage for the changes, that would be fantastic.

[ltvolks]

Here you go... Curiously, webob.util only shows 91% coverage against master when I run coverage locally, but Travis seems happy enough.

[digitalresistor]

I'll work on any other coverage issues once I get this pulled in, thanks for updating this PR with tests. :-)