Performance optimization isBefore/isAfter/isSame

[hupfis]

• optimized performance of isBefore/isAfter/isSame by using valueOf and
  not momentizing/cloning already existing moment objects
• 90% performance improvement when using without specific unit and the
  input is already a moment object

reviewed by PatrickBic

[ichernev]

OK, I approve the idea of the optimization.

Can you please

• rebase your patch on top of develop
• use a variable for input / moment(input) to reduce the cases in each function.
• use + in front of an expression in place of valueOf() (code style in the rest of the code)

[hupfis]

New pull request at #1892