Removing magic constants

...for increased readability of code.
Merge pull request #12 from JensRantil/remove-magic-constant

Removing magic constants
typo fix
Readme improvements

 * Percentage => percentile
 * Making it clear what types of buckets supports percentiles vs.
   sampling.
Clarifying that percentile threshold is 0-100
Merge pull request #15 from JensRantil/readme-update

Readme improvements
Merge pull request #17 from JensRantil/clarify-percent-threshold-parameter

Clarifying that percentile threshold is 0-100