Make expect(a).to be_within(x).percent_of(y) work with integer x and y

Signed-off-by: Sam Phippen <samphippen@googlemail.com>
Add changelog entry for integer be_within percent fix.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>
Merge pull request #229 from samphippen/percent-matcher-with-integers

Make the expect(a).to be_within(x).percent_of(y) matcher work with integer x and y