'exit' and 'quit' cause exit regardless of open block
Don't exit if 'exit' or 'quit' are in quote blocks
Merge pull request #434 from joequery/exit_in_string

Improve handling of "exit" and "quit" in mirb.
use bit field mrb_vtype in MRUBY_OBJECT_HEADER
rename MRUBY_OBJECT_HEADER to MRB_OBJECT_HEADER
replace mrb_iv_set by mrb_obj_iv_set
reduce mrb_funcall invocations
remove BEGIN/END from syntax
remove assignment to unused retval
remove assignment to unused dst; close #424
Modify argument of mrbc_filename.
allocf should take ud; https://twitter.com/junjis0203/status/236949976461221889
replace RiteVM in comments by mruby
mrb_open_allocf() should keep ud in mrb_state
Merge pull request #435 from masamitsu-murase/modify_mruby_filename

Modify argument of mrbc_filename.
Refer to mrb->ci instead of local variable `ci` after m->body.func is called,
because mrb->ci may be changed in m->body.func.
Refer to irep_capa as well as irep_len when irep is marked by GC.
Initialize mrb->irep array with 0 when realloc is called.
Merge pull request #436 from masamitsu-murase/modify_reference_to_ci

Modify reference to ci.
Merge pull request #437 from masamitsu-murase/modify_irep_initialization_and_gc_mark

Modify irep initialization and GC.
abort() if mrb->jmp is empty