Ternary operator, String concat, Parameter flexibility

Features

• Implemented ternary operator ( #12 and #17 )
• Implemented string concatenation ( #13 )
• Parameters can now be specified as structures other than map[string]interface{}. Anything that exposes a Get(string)(interface{}, error) method receiver can now be passed into Eval. ( #14 )

Enhancements

• Optimized regex compilation. When a regex is used against a constant string, that string is compiled at parsing-time, not evaluation-time. Also allowed a *regexp.Regexp object to be passed in as a parameter, and used as the pattern for a regex. Strings used as patterns are still supported, but will be compiled during each evaluation, and therefore slower. ( #16 )