Doc improvements

Note: enabling doctest revealed some errors in the code examples and in the code. I was able to fix some of them, but the following are still left:

```
**********************************************************************
File "README.rst", line 97, in README.rst
Failed example:
    interval = Interval([date(2000, 1, 1), inf])
Exception raised:
    Traceback (most recent call last):
      File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/doctest.py", line 1289, in __run
        compileflags, 1) in test.globs
      File "<doctest README.rst[26]>", line 1, in <module>
        interval = Interval([date(2000, 1, 1), inf])
      File "intervals/interval.py", line 571, in __call__
        step=step
      File "intervals/interval.py", line 140, in __init__
        self.parser(bounds, lower_inc, upper_inc)
      File "intervals/interval.py", line 182, in lower
        self._lower = self.round_value_by_step(value)
      File "intervals/interval.py", line 500, in round_value_by_step
        round((self.type(Decimal('1.0')) / self.step) * value)
    TypeError: unsupported operand type(s) for *: 'int' and 'datetime.date'
**********************************************************************
File "README.rst", line 98, in README.rst
Failed example:
    interval
Expected:
    DateInterval('[2000-1-1,)')
Got:
    FloatInterval('[1.5, 4]')
**********************************************************************
File "README.rst", line 100, in README.rst
Failed example:
    interval.type
Expected:
    date
Got:
    <type 'float'>
**********************************************************************
File "README.rst", line 266, in README.rst
Failed example:
    interval.upper
Expected:
    6
Got:
    0
**********************************************************************
1 items had failures:
   4 of  66 in README.rst
***Test Failed*** 5 failures.
```