Added power ratio units.

- Power ratio is the strength of a signal expressed in decibels (dB) relative to one watt.
- Power ratios are commonly used in audio, RF, and telecommunications engineering.
Merge pull request #48 from bretthysuik/feature/power-ratio

Feature: Power ratio

    Power ratio is the strength of a signal expressed in decibels (dB) relative to one watt.
    Power ratios are commonly used in audio, RF, and telecommunications engineering.
    Updated Readme with more accurate information about the number of units of measure and tests in this library.