Fixes #7: Allow user to pick extract to let prefix

  Add custom var 'ruby-refactor-let-prefix that
  gets used to prefix the variable in regions
  when extracting let
Fixes #6: Extract to Let prefixing more than label

 Turn on case senstitivy when doing the line by
 line string replace when extracting let region
Fixes #3: Extract to let errors w/o assignment line

 * Check for current line or first line of region to have an
   assignment before executing extract-to-let
 * Trim newlines off front and back of regions before using
 * Refactor ruby-refactor-extract-to-let to call methods for line and region
 * Adjusted some function names to be more elisp-y
 * Re-ordered a number of functions to better organize code