Avoid exception when unable to match UA

Make detect() return an empty object if it hasn't matched any known browser, so that graded browser support (and any external code using this library) doesn't raise a "Cannot read property ... of undefined" exception
Add IE11 support

Match IE by looking for either "MSIE" or "Trident"
Read the new "rv:x.y" version number
Merge pull request #20 from MartinodF/patch-1

Add IE11 support + avoid exception when unable to match UA