Support override of keys using dot notation.

+ This allows calls like `$captcha->setOption('text.colour', ...)` to set the `colour` subkey
  of the `text` top-level key.
+ Also changed `array_merge` to `array_merge_recursive` to allow nested keys to be passed to
  the constructor without clobbering sibling keys.
Support retrieval of keys using dot notation.

+ This allows `$captcha->getOption('text.colour')` to retrieve the `colour` sub-key
  of the `text` top-level key.
Merge pull request #4 from leftclickben/SupportNestedKeyOverride

Support nested key override