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 #8212
Closes #8193