fix requiring of core_ext when time-warp gem is already loaded

(this is probably the root cause of #3)

in ```ruby-mass.rb``` this line:
```
require 'core_ext'
```
will fail to load ```ruby-mass```'s ```core_ext.rb``` if the ```time-warp``` gem was loaded first:
```
2.1.0 :001 > require 'ruby-mass'
 => true
2.1.0 :002 > $".select{|a| a.match /\/lib\/core_ext\.rb/ }
 => ["/Users/schatzj/.rvm/gems/ruby-2.1.0@rails3-ruby21/gems/ruby-mass-0.1.3/lib/core_ext.rb"]
```
vs
```
2.1.0 :001 > require 'time-warp'
 => true
2.1.0 :002 > require 'ruby-mass'
 => true
2.1.0 :003 > $".select{|a| a.match /\/lib\/core_ext\.rb/ }
 => ["/Users/schatzj/.rvm/gems/ruby-2.1.0@rails3-ruby21/gems/time-warp-1.0.13/lib/core_ext.rb"]
```

this patch changes the requires in ```ruby-mass.rb``` to use fully qualified pathnames to work around this:
```
2.1.0 :001 > require 'time-warp'
 => true
2.1.0 :002 > require 'ruby-mass'
 => true
2.1.0 :003 > $".select{|a| a.match /\/lib\/core_ext\.rb/ }
 => ["/Users/schatzj/.rvm/gems/ruby-2.1.0@rails3-ruby21/gems/time-warp-1.0.13/lib/core_ext.rb", "/Users/schatzj/git/ruby-mass/lib/core_ext.rb"]
```