Fix setting of default keymap (#17).

defvar was being called again after the variable was already set,
which doesn't work. Also, there was a typo when redefining
it (ruby-reactor-mode-map instead of ruby-refactor-mode-map). I have
followed a pattern found in other ELPA packages, such as scala-mode2,
to fix it.
Make keymap prefix configurable.
Merge pull request #18 from dodecaphonic/master

Fix setting of default keymap (#17).