Version 0.2
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Features: 
* added a 'functionable' test; tests if the value can be
used by match.fun( ).
* added a 'variadic' test; tests if the function is variadic
* added support for negation traits; "!string", "!unary function" and 
c("!length_one string", "number") are now valid trait specifications.
* added a function 'add_tests' that allows new trait tests to be added to 
the environment containing trait tests.

Bug Fixes: 
* The errors of missing argument 'traits' or 'value' now correctly
reports which argument is missing.
* A clearer error is given when arguments are not given to require_a.


