[BUGFIX] Always escape backslashes in strings

Backslashes in PHP strings always need to be escaped as two backslashes
(even in regular expressions). There are only two exceptions:

* sequences like \n in double-quoted strings
* escaped single quotes in single-quoted strings (not present in Emogrifier)

Closes #123
[CLEANUP] Drop EmogrifierTest::tearDown

tearDown only contained unset($this->subject). As the unset is not
really necessary for helping the garbage collector, the complete
method can safely be removed.

Closes #131
Merge pull request #130 from jjriv/bugfix/backslashes

[BUGFIX] Always escape backslashes in strings
Merge pull request #132 from jjriv/cleanup/tearDown

[CLEANUP] Drop EmogrifierTest::tearDown
[TASK] Switch the coding style to PSR-2

Also update PHP_CodeSniffer to 2.0 as this version provides the
corresponding standard.

Closes #102