formalize function style and naming

1.  Top-level helper functions are defined via function statements:

        function _add(a, b) { return a + b; }

2.  Names of top-level helper functions are underscore-prefixed.

3.  Exported functions are referenced by alias when used internally:

        R.project = useWith(_map, pickAll, identity);

    In this example pickAll and identity are aliases for R.pickAll and
    R.identity respectively.

4.  Aliases for exported functions match property names of the R object.
    Aliases are not underscore-prefixed.

5.  Function expressions should be named rather than anonymous. These
    names should not be underscore-prefixed.
Merge pull request #506 from davidchambers/consistency

formalize function style and naming
ensure return value of isArray is Boolean