fix(ngSanitize): follow HTML parser rules for start tags / allow < in text content

ngSanitize will now permit opening braces in text content, provided they
are not followed by either an unescaped backslash, or by an ASCII letter
(u+0041 - u+005A, u+0061 - u+007A), in compliance with rules of the parsing
spec, without taking insertion mode into account.

BREAKING CHANGE

Previously, $sanitize would "fix" invalid markup in which a space preceded
alphanumeric characters in a start-tag. Following this change, any opening
angle bracket which is not followed by either a forward slash, or by an
ASCII letter (a-z | A-Z) will not be considered a start tag delimiter, per
the HTML parsing spec
(http://www.whatwg.org/specs/web-apps/current-work/multipage/parsing.html).

Closes #8193
As far as HTML parsing is concerned, `/</[a-zA-Z/` is the start of a tag, so we shouldn't "fix" this, I think