Fix sign_symbol_for with Rails 4.0.1

Uses the "cover?" method because "include?" is no longer allowed on DateTime in Rails 4.0.1 for performance reasons. This also increases performance!
Merge pull request #9 from travisp/patch-1

Fix sign_symbol_for with Rails 4.0.1
testing on 2.0 instead of 1.8.7
scope syntax fix
release 0.2.6
Get zodiac symbol by date