Modified __lt__ in Marker class.

[Mo2000]

Marker class in /voluptuous/schema_builder.py needed to have the __lt__() function
modified in order to compare against regular strings as well.

This will allow the usage of Voluptuous Optional objects as keys in dicts alongside
strings and int.

An example of this would be a dict as such:

{
    'Name': <function is_str at 0x1057b8b90>,
    'ExecutionTimestamp': <function is_str at 0x1057b8b90>,
    Optional('Classification'): [Any([<function is_str at 0x1057b8b90>, <function is_null at 0x1057b8aa0>])]
}

Sorting the above dict without the proposed changes will result in an error similar to the one below:

  File "/Users/mo2000/Work/QA/venv/lib/python2.7/site-packages/voluptuous/schema_builder.py", line 902, in __lt__
    return self.schema < other.schema

[coveralls]

Coverage decreased (-0.09%) to 95.17% when pulling 826c4ae on Mo2000:master into a206303 on alecthomas:master.

[coveralls]

Coverage increased (+0.009%) to 95.265% when pulling f170fa7 on Mo2000:master into a206303 on alecthomas:master.

[alecthomas]

Can you add a test for this please?

[Mo2000]

Test has been added

[alecthomas]

Thanks!