Made all operators constexpr.
Made the double() operator a constexpr as well.
Removed `constexpr` from operator+ and operator- as they use += and -= which are not allowed in `constexpr` functions.
Added a file to test constexpr usage.
Updated to create exceutable for the contexpr test.
Added tests for all constexpr methods and functions.
Added more derived units from NIST site.

Added all derived units from Table 2 at
http://physics.nist.gov/cuu/Units/units.html.
Added more derived units based on NIST tables and articles on Wikipedia.
Merge pull request #7 from crayzeewulf/derived_units

Added derived units