Fixed a invalid source path name on mruby debugger.

The change removes several internal instance variables used by
class name management. The variables `__classid__` and `__classpath__`
are no longer available. `__outer__` is used only for unnamed outer
classes/modules (and will be removed after they are named).

[Important note]
Along with this change we removed several public functions.

 - mrb_class_outer_module()
 - mrb_class_sym()

We believe no one have used those functions, but if you do, please
ask us for the workaround.

From `mrb_mod_const_set` to `mrb_const_set`.

This update fix CRuby scoping incompatibility; close #600; close #3200

Updated AUTHORS

Improved speed of enumeration methods

Merge branch 'move-task-class-definitions' of https://github.com/udzura/mruby into udzura-move-task-class-definitions

Extended Module#const_get to support class paths

Added Array#bsearch_index

Fixed return value of Module#include and #prepend

It reduces the memory consumption and sometimes improve the
performance as well.  For example, the consumed memory size
of `bench/bm_ao_render.rb` is reduced from 1.2GB to 1GB, and
its total execution time become 18.795 sec from 22.229 sec.

Fixed setting custom rbfiles in gemspec