Improvement to CRuby like Regexp.

Following features were added:
* support *$;* default splitter
* support group name back referencing in *[]*, *beg*, *end*, *offset* method
* make GC manage *OnigRegion** freeing
* support regexp global variables used in CRuby
* add class methods to disable regexp global variables
  * *When disabled some object allocation would be delayed or wouldn't happen.*
* add *escape* class method to escape regexp string
* use constants defined in *oniguruma.h* instead of literals