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

WIP: Port into v1.2.x (needed some fixes to the jshint rules to match master)

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).