add assert_{true,nil,equal} methods.

 - add assert_equal, assert_true, assert_nil method to test/assert.rb

sample test code:
```ruby
assert('String#gsub', '15.2.10.5.18') do
  assert_equal('aBcaBc', 'abcabc'.gsub('b', 'B'))
  assert_equal('aBcaBc', 'abcabc'.gsub('b'){|w| w.capitalize }, 'gsub with block')
  assert_equal('$a$a$',  '#a#a#'.gsub('#', '$'), 'mruby/mruby#847')
  assert_equal('$$a$$',  '##a##'.gsub('##', '$$'), 'mruby/mruby#847 another case')
end
```

result:
```
mrbtest - Embeddable Ruby Test

This is a very early version, please test and report errors.
Thanks :)

F
Fail: String#gsub [15.2.10.5.18]
 - Assertion[4] Failed: mruby/mruby#847 another case
    Expected: "$$a$$"
      Actual: "$$a"
Total: 1
   OK: 0
   KO: 1
Crash: 0
 Time: 0.00132 seconds
```