add {Array|Hash|String}.try_convert

Fix MRB_DISABLE_STDIO typo.

Support Numeric#zero? and Numeric#nonzero?

Fix typo on MRB_DISABLE_STDIO.

Fix check for VisualStudio Version in mruby-math gem

Bugfix nagative-number lshift() bit overflow

Update README.md

Fix mrbgems doc link.

Enable full-debug build on travis using `MRB_ENABLE_DEBUG_HOOK`

Fix MRB_ENABLE_DEBUG_HOOK config typo left from 4440566b

/cc #3014

add RUBY_ENGINE_VERSION

document MRB_UTF8_STRING config macro

fix MRB_DISABLE_STDIO typos

RUBY_ENGINE_VERSION is equivalent to MRUBY_VERSION. It would be a
standard way to get the interpreter version (without a case expression).

It's also already defined by CRuby 2.3, JRuby 9000, Opal, and Rubinius.

changes:
 * rename DISABLE_STDIO -> MRB_DISABLE_STDIO
 * rename ENABLE_DEBUG -> MRB_ENABLE_DEBUG_HOOK
 * no more opposite macro definitions (e.g. ENABLE_STDIO, DISABLE_DEBUG).
 * rewrite above macro references throughout the code.
 * update documents

Add contributors from mruby-cli project to AUTHORS file

Added type annotation to YARD.

fix comment

This patch changes git gem behavior to never pull gems unless specified.