[TASK] Drop support for PHP 5.3.

PHP 5.3 had its end of life on August 14th, 2014:
http://php.net/supported-versions.php

So Emogrifier now requires PHP 5.4 or greater.

Closes #79
[CLEANUP] Drop the default values for setHtml and setCss

Calling these methods without any parameters just does not make
any sense.

Closes #115
[TASK] Add HHVM to the travis build matrix

Closes #108
[TASK] Update PHPUnit to 4.4.*

Closes #118
[CLEANUP] For @var PHPDOc comments, put the type first

This makes the order consistent with what PhpStorm uses by default.

Closes #112
[CLEANUP] Use the Type[] array syntax in PhpDoc

This will help IDEs like PhpStorm with static code analysis and
code completion.

Also add missing blank lines between the @param and @return lines.

Closes #104
Merge pull request #114 from jjriv/task/remove-php-5-3

[TASK] Drop support for PHP 5.3.
Merge pull request #116 from jjriv/cleanup/default-values

[CLEANUP] Drop the default values for setHtml and setCss
Merge pull request #117 from jjriv/task/hhvm

[TASK] Add HHVM to the travis build matrix
Merge pull request #119 from jjriv/task/phpunit-update

[TASK] Update PHPUnit to 4.4.*
Merge pull request #120 from jjriv/cleanup/var-order

[CLEANUP] For @var PHPDOc comments, put the type first
Merge pull request #121 from jjriv/cleanup/array-syntax

[CLEANUP] Use the Type[] array syntax in PhpDoc
[WIP][TASK] Switch the coding style to PSR-2

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

Closes #102