Numeric Bucketing

Thanks for a great project! I've added some bucketing functions I wrote to make working with Localytics a bit easier.

Many analytics services (*cough* Localytics *cough) do not bucket your values for you, so your reported numeric data can be awfully difficult to read.

These bucketing functions will accept a number and output a bucketed range as a string.

Example: A value of 47, with bucket size 25, will return the string `"25-49"`.
A value of 10, with bucket size 15, will return the string `"< 15"`.

Currently implemented as a category on `AnalyticsKit`; open to other suggestions. Thanks!

Edit: Looks like the Localytics subspec has been omitted from the podspec. Intentional?