Added assertions for NaN

Use
x.should.be.NaN;
or
x.should.not.be.NaN;
isNaN now rejects all non-numbers
Fix examples to work again
Changed Expresso example to Mocha.
Merge pull request #128 from gabrielf/mocha-example

Changed Expresso example to Mocha.
minor typo
Merge pull request #131 from fredr/readme-typo

fixes minor typo in readme
Merge branch 'master' of github.com:visionmedia/should.js into v2

Conflicts:
	Readme.md
Expose inspect function, that people can change options as they want.
Merge branch 'nan-assertions' of https://github.com/ichernev/should.js into v2
Added Ininity assertion, fixed test for NaN