Modifying core types should be optional

[kmcphillips]

This is a solid library. I can't find any unit tool in ruby that touches this one.

Unfortunately we can't bring it into our project because this gem opens up and modifies a number of core types: Array, Date, Fixnum, Math, Numeric, Object, String, Time

This is bad style and is dangerous. Is there a reason these core type modifications were made mandatory? They should be opt-in similar to how namespacing is done. These core extensions are depended on for internal implementation as well so I can't even selectively include units and conversions without modifying all those above classes.

[olbrich]

Some of these core modification are necessary and several are not. I plan to make this a lot more configurable, but have not yet had a chance to implement the required changes. Stay tuned.

[kreintjes]

Fixed with #157