Add a new gem: `mruby-catch`.

Implements `catch`/`throw` non-local jump inherited from Lisp.

`catch([tag]) {|tag| block }  -> obj`

Example:
```
  catch(:foo) { 123 }               # => 123
  catch(:foo) { throw(:foo, 456) }  # => 456
  catch(:foo) { throw(:foo) }       # => nil
```

mrbgems/mruby-catch/mrbgem.rake

+MRuby::Gem::Specification.new('mruby-catch') do |spec|
+  spec.license = 'MIT'
+  spec.author  = 'mruby developers'
+  spec.summary = 'Catch / Throw non-local Jump'
+end

mrbgems/mruby-catch/mrblib/catch.rb

+class ThrowCatchJump < Exception
+  def initialize(tag, val)
+    @tag = tag
+    @val = val
+    super("uncaught throw :#{tag}")
+  end
+  def _tag
+    @tag
+  end
+  def _val
+    @val
+  end
+end
+
+module Kernel
+  def catch(tag, &block)
+    block.call(tag)
+  rescue ThrowCatchJump => e
+    unless e._tag == tag
+      raise e
+    end
+    return e._val
+  end
+  def throw(tag, val=nil)
+    raise ThrowCatchJump.new(tag, val)
+  end
+end