Implement the class function

[MichaelDao]

As this is my first pull request to this project, I thought it would be good to improve the documentation so that when someone new like me starts, they can read the same documentation everyone else has read and learn more about the IEEE 754 revised floating point.

I implemented the class function (more info: http://speleotrove.com/decimal/damisc.html#refclass).
A miscellaneous function that takes in a decimal and returns a string of 10 possibilities:

• "sNaN" (signaling NaN)
• "NaN" (quiet NaN)
• "-Infinity" (negative infinity)
• "-Normal" (negative normal finite number)
• "-Subnormal" (negative subnormal finite number)
• "-Zero" (negative zero)
• "+Zero" (non-negative zero)
• "+Subnormal" (positive subnormal finite number)
• "+Normal" (positive normal finite number)
• "+Infinity" (positive infinity).

Also made a minor change to printout from the test suite to space out the logging.

[MichaelDao]

Had code reviewed by @joshcarp in person and have applied the suggestions:

• Fixed the issues picked up by gometalinter.
• Refactored the class function to concatenate "+" or "-" onto the returning string.
• Unpacked the decimal into decparts and utilised private functions to avoid having to unpack it each time with public functions.
• Created more private functions e.g. isSubnormal() for use with decparts, reorganized them so that they are next to other relevant private functions

[MichaelDao]

I have made all the recommended changes suggested by @anzdaddy

• The code in the documentation has been given syntax highlighting as well as minor fixups.
• The comment for the class function is now complete, along with a link reference to more details.
• Quickly fixed the way we pass parameters to the Class() function, is now matching the function in the documented standard

[anzdaddy]

(Damn, I've got to stop doing reviews as @marcelocantos accidentally!)

[joshcarp]

Some changes still needed, Looks alright though

[joshcarp]

Hey missed the pointer receiver thing. Change all the decParts methods from
func (dec decParts) name()
to
func (dec *decParts) name()
This means that the decParts instance won't be copied into the local namespace, but will use the existing memory allocation of dec
This means that its faster, but also means that the dec is mutable

[joshcarp]

Remove link

[joshcarp]

Slight pr creep but all good,
There seems to be some more problems with using outdated methods, but that'll be fixed in the next PR