Accept header normalization

According to RFC 2616, the mimetype in the accept header should be matched in a case insensitive way.

While fixing the root case insensitivity in webob (https://github.com/Pylons/webob/pull/56), I noticed a glitch in view registration: if you run pyramid_view_accept.py with an unpatched pyramid, both the view callables get registered; which one will be called depends on the client-supplied Accept header, and the webob available version...

```
$ python pyramid_view_accept.py
Route: service, Path: /, Callable: service_HTML, Accept: text/HTML
Route: service, Path: /, Callable: service_html, Accept: text/html
Listening on port 6543
Accept: text/html
localhost.localdomain - - [08/Jun/2012 18:05:41] "GET / HTTP/1.1" 200 92
Accept: text/HTML
localhost.localdomain - - [08/Jun/2012 18:10:39] "GET / HTTP/1.0" 200 92
```

With the proposed patches, the configurator would notice the attempted shadowing and correctly raises a ConfigurationConflictError:

```
$ python pyramid_view_accept.py
Traceback (most recent call last):
  File "pyramid_view_accept.py", line 38, in <module>
    app = config.make_wsgi_app()
  File "/home/lorenzo/pyramid/pyramid/config/__init__.py", line 921, in make_wsgi_app
    self.commit()
  File "/home/lorenzo/pyramid/pyramid/config/__init__.py", line 594, in commit
    self.action_state.execute_actions(introspector=self.introspector)
  File "/home/lorenzo/pyramid/pyramid/config/__init__.py", line 1030, in execute_actions
    for action in resolveConflicts(self.actions):
  File "/home/lorenzo/pyramid/pyramid/config/__init__.py", line 1136, in resolveConflicts
    raise ConfigurationConflictError(conflicts)
pyramid.exceptions.ConfigurationConflictError: Conflicting configuration actions
  For: ('view', None, '', None, <InterfaceClass pyramid.interfaces.IView>, None, None, ('GET', 'HEAD'), 'service', None, False, 'text/html', None, None, None)
    Line 12 of file pyramid_view_accept.py:
        accept="text/HTML")
    Line 18 of file pyramid_view_accept.py:
        accept="text/html")
```

pyramid_view_accept.py:
```python
#!python
# encoding: utf-8

from pyramid.config import Configurator
from wsgiref.simple_server import make_server
from pyramid.view import view_config
from pyramid.response import Response

import sys

@view_config(route_name='service', request_method='GET',
             accept="text/HTML")
def service_HTML(request):
    return Response('<html><head><title>HTML SERVICE</title></head><body><p>route_name=service</p></body></html>\n')

@view_config(route_name='service', request_method='GET',
             accept="text/html")
def service_html(request):
    return Response('<html><head><title>html service</title></head><body><p>route_name=service</p></body></html>\n')


if __name__ == '__main__':
    # configuration settings
    if len(sys.argv) > 1:
        httpport = int(sys.argv[1])
    else:
        httpport = 6543
    settings = {}
    settings['reload_all'] = True
    settings['debug_all'] = True
    # configuration setup
    config = Configurator(settings=settings)
    config.add_route(name='service', pattern='/')
    config.scan()
    # serve app
    app = config.make_wsgi_app()
    # print view mappings
    rt_map = config.get_routes_mapper()
    views = config.introspector.get_category('views')
    for view in views:
        _callable = view['introspectable']['callable'].__name__
        _accept = view['introspectable']['accept']
        _route_nm = view['introspectable']['route_name']
        _route = rt_map.get_route(_route_nm)
        if _route:
            print 'Route: %s, Path: %s, Callable: %s, Accept: %s' % (_route.name, _route.path, _callable, _accept)
    server = make_server('', httpport, app)
    print 'Listening on port %i' % httpport
    server.serve_forever()
```