Set resizable to false
Disable convert button after it's clicked

This prevents a scenario where a user can enter a value and convert it
to a new value containing a comma (Ex: 5 pounds -> 2,272.727 grams) and
then click convert again causing a number format exception
Enable convert button after changing measurement
Added some power units
Added pressure units
Merge pull request #1 from jtf323/master

Added power and pressure