Addition of a 'startCodepoint' option to determine where the unicode code points should start from.

Created as a result of a nasty problem we've encountered with IE8 whereby rendering unicode private use area glyphs in some circumstances results in the browser falling back to IE7 compatibility mode.

The solution in this case was to generate glyphs in the normal latin alphanumeric range.

Have added a couple of test cases to make sure that:
- glyphs are created in the SVG font from the specified codepoint
- the non-default codepoints are correctly output in the generated CSS file

If the 'startCodepoint' option is not provided, the usual default of '0xE001' applies.