Add respondsTo and satisfies as aliases

[couchand]

This pull requests adds the assertion alias respondsTo for respondTo and satisfies for satisfy. This is to complete the API for subordinate clause grammar, such as:

expect(myObject).to.have.property('child').that.respondsTo('toString');
expect(myObject).to.have.property('child').that.satisfies(myPredicate);

When adding this I noticed one other place the API was incomplete: there is no exists property. This seems reasonable, though, since you can only get to a subordinate clause by previously asserting the existence of something.

Are there docs that I should update for this as well?

[keithamus]

Thanks @couchand! You've already updating the docs, by virtue of adding the @alias directives in the docblocks!

[couchand]

🍻