Comply with PSR-2 and Symfony conventions

Coding style is changed to comply with:
- [PSR-2 conding style](http://www.php-fig.org/psr/psr-2/)
- [Symfony Coding Standard](http://symfony.com/doc/current/contributing/code/standards.html)
- [Symfony Naming Conventions](http://symfony.com/doc/current/contributing/code/conventions.html)

Resolves #13.

Previous interface is left, but deprecated. Tests for deprecated methods are also left so the old interface is kept valid unless explicitly decided to be removed.

---

PHP Code Sniffer currently complains with the following warnings:

```
FILE: src/Openbuildings/Postmark/Api.php
--------------------------------------------------------------------------------
FOUND 3 ERROR(S) AFFECTING 3 LINE(S)
--------------------------------------------------------------------------------
 203 | ERROR | Method name "Api::is_secure" is not in camel caps format
 215 | ERROR | Method name "Api::set_secure" is not in camel caps format
 226 | ERROR | Method name "Api::get_send_uri" is not in camel caps format
--------------------------------------------------------------------------------


FILE: src/Openbuildings/Postmark/Swift/PostmarkTransport.php
--------------------------------------------------------------------------------
FOUND 1 ERROR(S) AFFECTING 1 LINE(S)
--------------------------------------------------------------------------------
 15 | ERROR | Class name "Swift_PostmarkTransport" is not in camel caps format
--------------------------------------------------------------------------------


FILE: src/Openbuildings/Postmark/Swift/Transport/PostmarkTransport.php
--------------------------------------------------------------------------------
FOUND 2 ERROR(S) AFFECTING 2 LINE(S)
--------------------------------------------------------------------------------
  13 | ERROR | Class name "Swift_Transport_PostmarkTransport" is not in camel
     |       | caps format
 217 | ERROR | Method name
     |       | "Swift_Transport_PostmarkTransport::convert_email_array" is not
     |       | in camel caps format
--------------------------------------------------------------------------------


FILE: tests/src/Swift/PostmarkTransportTest.php
--------------------------------------------------------------------------------
FOUND 1 ERROR(S) AFFECTING 1 LINE(S)
--------------------------------------------------------------------------------
 12 | ERROR | Class name "Swift_PostmarkTransportTest" is not in camel caps
    |       | format
--------------------------------------------------------------------------------


FILE: tests/src/Swift/Transport/PostmarkTransportTest.php
--------------------------------------------------------------------------------
FOUND 1 ERROR(S) AND 2 WARNING(S) AFFECTING 3 LINE(S)
--------------------------------------------------------------------------------
  18 | ERROR   | Class name "Swift_Transport_PostmarkTransportTest" is not in
     |         | camel caps format
 191 | WARNING | Line exceeds 120 characters; contains 1600 characters
 211 | WARNING | Line exceeds 120 characters; contains 1600 characters
--------------------------------------------------------------------------------
```

The legacy methods (which are in snake_case) are left intentionally to keep the old interface working.

The left long lines are base64 encoded images and do not do any harm to the readability.

The classes are left the same for backwards compatibility, but I'd take any suggestions on that.