Use bc_math for floating point calculations for reliable results

Ref: issue #14
2 more bc_math calculations + enriching of documentation according to the phpDoc standards
Efficiency: only test for extensions loaded once & hold result in class property
Mini-tweaks
Comment whitespace
*sigh* ... using $this in static context... fixed
Efficiency tweak on the clean_text() method
Merge pull request #16 from jrfnl/master

Use bc_math for precision calculations - fixes issue #14