Doesn't support Python 3

Today I tried to check my files with flakeplus using python 3.5.2 and received this errors:

```
if isinstance(value, basestring):
NameError: name 'basestring' is not defined
```
and

```
for k, v in vars(options).iteritems()
AttributeError: 'dict' object has no attribute 'iteritems'
```

