Optimize and extract multiplication and division operations on Fixnum and Float

Signed-off-by: Veselin Vasilev <vesselinv@me.com>
Redid if statements into cases. Added missing tests
Merge branch 'vesselinv-optimize-mul-and-div'

PR #8 by Veselin Vasilev: Optimize and extract multiplication and division operations on Fixnum an...

* Directly return the original method call if argument is a Numeric: when performing millions of multiplication/division operations on Fixnum/Float, the many `Kernel#kind_of?` calls slow down performance dramatically.
* Replaced `#kind_of?` with `#is_a?`: benchmarking the two proves that the latter is slightly faster
* Extracted method definitions in a module
Amending PR #8 by @vesselinv. (e47a427)

* `lib/gsl/oper.rb`:
  * Include `Numeric` in case statement.
  * Avoid local variable for GSL classes.
  * More specific names for aliases.
  * Formatting.

* `test/gsl/oper_test.rb`:
  * Prefer `assert_in_epsilon` over too broad `assert <=`.
  * Formatting.

* `rb-gsl.gemspec`:
  * Will be updated by `gem:spec:update` task.