Raise an error when someone attempts to aim() at a percent based strategy

This change disables new uses of the percent and percent_visited strategies and also introduces the new aliases percent_drecated and percent_visited_deprecated.

The goal is to switch all current uses of the percent based strategies over to the *deprecated alias names after this change goes out. This will enable us to follow up with another commit that corrects the implementation of the percent based strategies without affecting the set of users that are exposed to the deprecated implementation of the strategies.

We hope to re-implement the percent based strategies in a way that can be used safely with experiment analysis. The current implemention tends to distribute features using the percent based strategies over the same subsets of users making it difficult to draw conclusions from the results of the analysis of those features.

Testing:
I wrote new specs and verified that all specs passed
Re-implement Trebuchet percent strategy for better experiment analysis

The original Trebuchet percent strategy was not good for independent feature analysis because it tended to cluster percent launches for features to the same subsets of users.

This new strategy is better because it computes a hash of the feature name along with the identifying user or visitor id and therefore distributes features randomly across their domains. This makes it more likely that the results of analysis for individual features are not biased by other features that were enabled with the percent strategy at the same time.

Note that I exliminated the :to / :from options from the percent strategy since they are no longer as useful with the new implementation and seemed like interface bloat.

Testing:
Re-ran specs and implemented new specs for the new percent strategy
Eliminate dead code that I missed in my last commit
Fix export in strategies that use Percentable

I skrewed this up in my previous commit by still referencing the no longer present @to instance variable.
Use be_close to compare to target percent

This captures the goal of the spec more than the more exact comparison did.
Merge pull request #8 from airbnb/reimplement-percent-strategy

Reimplement percent strategy