Use simpler way to set model name

The method signature for ActiveModel::Name.new changed between
version 3.0.9 and 3.1.0. Because of that, this test was failing for
ActiveModel < 3.1.0.
Handle nil time zones

It's possible to have Time.zone available but unconfigured, in which
case it is nil.
Fix a couple minor code issues
Fix a couple typos
Keep lines under 80 characters

We don't have a style guide, but I like to follow bbatsov's:
https://github.com/bbatsov/ruby-style-guide
Tidy up spec names
Fix minor spacing issues
Clean up method_missing and friends

In particular, filter_attr_accessor was kind of getting out of hand.

I don't like how the setup helpers all take almost the same arguments.
Check for the correct method
Fix #23; support strptime format strings
Extract common code between date/time filters
Merge branch 'master' into 0.2.0

Conflicts:
	lib/active_interaction/filters/date_time_filter.rb
	lib/active_interaction/filters/time_filter.rb