Prevent scope lookup from accidentally matching callable builtins

[echuber2]

Fixes #117

The current lookup by scope has false positives when the variable in the template coincidentally matches some Python builtin on the type. For example, {{title}} matches str.title when it shouldn't. This fix detects when the attribute found in the scope is callable and raises an exception to continue as normal. This fix does not break the intended ability to have callable lambdas in the template.

Before the fix:

>>> import chevron ; chevron.render('{{#title}}{{title}} abc{{/title}}', {'title': 'bob'})
'<built-in method title of str object at 0x7f2dadea95f0> abc'

(Here, it found a 'title' attribute on 'bob' and got messed up.)

After the fix:

>>> import chevron ; chevron.render('{{#title}}{{title}} abc{{/title}}', {'title': 'bob'})
'bob abc'

Callable functionality is still working fine:

>>> import chevron ; chevron.render('{{#bar}}{{foo}} abc{{/bar}}', {'foo': 'bob', 'bar': lambda text,render: render("rendered:"+text)})
'rendered:bob abc'

The lambda example from the chevron readme still works as before too.