Ensure space is inserted after link if cursor at end of <a> tag

If your cursor is at the end of a link, ensure pressing space always inserts
the space outside of the <a> tag.
Fix inserting line break at start of link

Before, hitting enter at the start of the link would result in the link text
being removed from an <a> on the new line, and an empty <a> tag on the previous
line. Now the link remains on the new line as expected, and no empty <a> tag is
produced.

in the preceding block.
Clean up empty inlines when deleting selection.

Fixes #6
Remove last remnants of IE8 support.
Replace feature test w/ UA sniff for buggy splitText. *sigh*

The feature test is sometimes passing in IE9, but then the buggy behaviour
still appears later. FFS IE.
Take over selection deletion in more cases.

Never really want to let the browser delete a selection; it always makes a mess
of it. This now covers the case when an English letter key or number key is
pressed when a selection exists. Handling this completely (to include all keys
that produce a symbol) is tricky due to cross-browser key event compatibility
issues.
Fix deleting selection across block boundary.
Fix selection lost on action in Presto
Record undo checkpoint before deleting any selection.