etc.c: small cleanup (delete unused functions)

mrb_data_object_alloc: unnecessary cast
mrb_lastline_get: mruby doesn't support $_, weird and unused code
mrb_exec_recursive: see 4e46abb86914b36e70b5f3ad0826d0b648e9b4ef
mrb_block_proc: unused, doesn't really do anything (or needs to be
rewritten)
mrb_obj_to_sym: actually use id (and MSVC detected unreachable code)
Happy new year! (update copyright year)

Let's not wait until May this time
(f0a9b95af3d542ac1db7fcb4a3d77990a284c185)!
mruby.h: remove mrb_str_format declaration

The function mrb_str_format is only defined when the mrbgem mruby-sprintf was
activated. That might not always be the case (by default it currently is).
mrb_str_format still has external linkage, so from now on mrbgem authors
have to declare it in their source code if they want or need to call it.
clean up external symbols

remove unused and unneeded:
- sysexit_status
- type (a global variable)

add mrb_ prefix to:
- codedump_all
- class_instance_method_list
- parser_dump

make various functions static, incl.:
- yyparse
- make_exception
add missing declaration of mrb_str_format
reindent parse.y
Merge pull request #1670 from cremno/remove-mrb_str_format-declaration

remove mrb_str_format declaration
Merge pull request #1668 from cremno/etc-c-cleanup

etc.c: small cleanup
Merge pull request #1669 from cremno/copyright-2014

update copyright year
Merge pull request #1671 from cremno/extern-cleanup

clean up external symbols
remove conflict
rework mruby-objectspace and gc.[ch]

- functions should have C linkage (for C++ code)
- add prefix to each_object_callback
- use more appropriate variable types / initialization
- ObjectSpace::count_objects has 1 opt. arg.
- prefer mrb_intern_lit to mrb_intern_cstr for str. lit.
- mruby/value.h is included by mruby.h
- adjust coding style
Merge pull request #1673 from cremno/rework-objspace-gc

rework mruby-objectspace and gc.[ch]
fix NODE_SPLAT codegen
Merge pull request #1675 from h2so5/node-splat-codegen

fix NODE_SPLAT codegen
add Float#nan?
Merge pull request #1678 from iij/float-nan-p

add Float#nan?