Proc class define before a first method

mrb_define_method setting mrb->proc_class for method proc
arity for cfunc is not implemented yet
Fix document of doc/mrbconf/README.md
Fix a document in doc/mrbgems/README.md
Fix document of  doc/compile/README.md
Merge pull request #2410 from suzukaze/Fix-docs

Fix document of doc/mrbconf/README.md
Merge pull request #2411 from suzukaze/Fix-docs2

Fix document in doc/mrbgems/README.md
Merge pull request #2412 from suzukaze/Fix-doc3

Fix document of  doc/compile/README.md
Update documentation of `summary` property of mrbgem spec.
Quote required mrbgem spec property.
Merge pull request #2413 from take-cheeze/summary_doc

Update documentation of `summary` property of mrbgem spec.
Merge pull request #2399 from ksss/cfunc-proc

Proc class should be defined before a first method definition
Merge remote-tracking branch 'upstream/master' into ext