Documentation and Examples for BigMath ext module

Pretty much what the commits say. I removed a line that should not be on BigMath's documentation (http://www.ruby-doc.org/stdlib-1.9.3/libdoc/bigdecimal/rdoc/BigMath.html) see top of that page,  'mathematical functions' should not be there. 

Also added a bunch of examples of usage and formatted method parameters on the existing doc.

Removed as BigMath is documented on lib/bigdecimal/math.rb
Documentation for BigMath module