Add HTML entity replacements for tab and space characters

[LaurentAerens]

diff_prettyHtml currently doesn't highlight leading/trailing space differences. Because spaces in HTML aren't visible.

This pull request changes that.

Instead of

<ins style="background:#e6ffe6;"> </ins>

diff_prettyHtml now returns

<ins style="background:#e6ffe6;">&nbsp;</ins>

resulting in differences with leading/training spaces being visible in HTML.

Example:
When we compare "Test" and "Test " (notice the second string doesn't equal the first string since it has a trailing space).

var diff = dmp.diff_main("Test", "Test "); // this results in the following diff: [[0, 'Test'], [1, ' ']]

Previously looked like this:

Now with this pull requests it looks like this:

Note: this issue also seems to be present in the JavaScript implementation: google#144

[LaurentAerens]

Is there any potential this PR could get a review?