Remove lint issues and duplicate tests from Base's unit tests
Add static `modifyAttrs()` method to BaseCore and Base

This method allows the `ATTRS` of an already defined BaseCore or Base
subclass to be safely modified. It will clear the cached class hierarchy
data so that the next instance of the class which is created will cause
the aggregated `ATTRS` to be re-computed.

A reference for this method is also added to every class created via
`Y.Base.create()`. This makes it more convenient to use and more
analogous to:

    // Before:
    MyClass.ATTRS.foo.value = 'foo';

    // Now:
    MyClass.modifyAttrs({
        foo: {value: 'foo'}
    });
Merge pull request #1 from ericf/attrs-modify

Add static `modifyAttrs()` method to BaseCore and Base