Configure rubocop rules for `dydx` lib.
Modify `dydx` lib with rubocop rules.
Configure rubocop rules for `dydx` spec.
Modify `dydx` spce with rubocop rules.
Modify Rakefile and dydx.gemspec with rubocop rules.
Merge remote-tracking branch 'upstream/master' into fit_rubocop

Conflicts:
	lib/dydx/algebra.rb
	lib/dydx/algebra/inverse.rb
	lib/dydx/algebra/operator.rb
	lib/dydx/algebra/set/base.rb
	lib/dydx/algebra/set/cos.rb
	lib/dydx/algebra/set/e.rb
	lib/dydx/algebra/set/fixnum.rb
	lib/dydx/algebra/set/float.rb
	lib/dydx/algebra/set/log.rb
	lib/dydx/algebra/set/num.rb
	lib/dydx/algebra/set/pi.rb
	lib/dydx/algebra/set/sin.rb
	lib/dydx/algebra/set/symbol.rb
	lib/dydx/algebra/set/tan.rb
	lib/dydx/integrand.rb
	lib/dydx/version.rb
	spec/dydx_spec.rb
	spec/lib/algebra/set/cos_spec.rb
	spec/lib/algebra/set/e_spec.rb
	spec/lib/algebra/set/fixnum_spec.rb
	spec/lib/algebra/set/float_spec.rb
	spec/lib/algebra/set/log_spec.rb
	spec/lib/algebra/set/num_spec.rb
	spec/lib/algebra/set/pi_spec.rb
	spec/lib/algebra/set/sin_spec.rb
	spec/lib/algebra/set/symbol_spec.rb
	spec/lib/algebra/set/tan_spec.rb
	spec/lib/delta_spec.rb
	spec/lib/function_spec.rb
	spec/lib/integrand_spec.rb
Improve rubocop rules on .rubocop.yml.
Remodify `dydx` spec with rubocop rules.
Reimprove rubocop rules on .rubocop.yml.
Remodify `dydx` lib with rubocop rules.
Add `rubocop` gem to Gemfile.
Restore `/` around spaces.

Restored spaces are only differential operator like `dydx`.
Division operator aren't restored.
Merge pull request #26 from gogotanaka/fit_rubocop

Fit to rubocop rules