Add checkers isTrue and isFalse.

Add checkers IsTrue & IsFalse.

Use :
```
c.Assert(0 < 1, IsTrue)
c.Assert(0 > 1, IsFalse)
```
instead of:
```
c.Assert(0 < 1, Equals, true)
c.Assert(0 > 1, Equals, false)
```