Implements precision on Decode and Encode
Implemented precision control as suggested by pull request #2

 - Added late static binding for precision
 - Moved precision unit test to isolated file test/PrecisionTest.php
 - Re-wrote usage & example on Readme

See:
Discussion: https://github.com/emcconville/google-map-polyline-encoding-tool/pull/2
Original implementation: https://github.com/rodo/google-map-polyline-encoding-tool/commit/5d86af3668bb511dff544e979965490ca0554699
Removed EOL whitespace, and added additional caution in doc
Added optional namespace build option in makefile. This will encapsulate the class under the 'emcconville' namespace.

e.g.
    make namespace