make mathn optional; include it explicitly for normal behaviour

Conflicts:
	VERSION
	lib/ruby_units/unit.rb
fix remaining issues with tests; allow Complex to be tested independently of mathn
refactor ruby units so it's redefinition of Fixnum./() doesn't leak

Conflicts:
	VERSION
	ruby-units.gemspec
fix the build under 1.9 and jruby

Conflicts:
	VERSION
	ruby-units.gemspec
slightly neater solution

Conflicts:
	VERSION
	ruby-units.gemspec
add a test for Math.sqrt returning a float
fix large bug when mathn is not required because of Fixnum division

Conflicts:
	VERSION