extend methods of decimal module

[Chris Angelico]

On Fri, Feb 28, 2014 at 1:15 PM, Mark H. Harris <harrismh777 at gmail.com> wrote:
> Its just easier to type D(2.78)  than Deciaml('2.78').

It's easier to type 2.78 than 2.718281828, too, but one of them is
just plain wrong. Would you tolerate using 2.78 for e because it's
easier to type? I mean, it's gonna be close.

Create Decimal values from strings, not from the str() of a float,
which first rounds in binary and then rounds in decimal.

ChrisA