[CLEANUP] Remove the changelog from the PHP file.

The changelog now resides in its own file in the Git repository.

Hence, we do not need to duplicate and maintain it in the PHP file.
[FEATURE] Add some basic unit tests.

Also add a BadMethodCallException if emogrify is called with empty HTML.
[TASK] Add a .gitignore.
[CLEANUP] Use the existing setters in the constructor.
[TASK] Copy information from the Pelago site into the Readme.

There is some information on Emogrifier at
http://www.pelagodesign.com/sidecar/emogrifier/ .

These information should also be available in the README.md.
[TASK] Document the usage.
[CLEANUP] Clean up the spacing around curly braces.
Merge pull request #15 from oliverklee/cleanup/remove-changelog

[CLEANUP] Remove the changelog from the PHP file.
Merge pull request #16 from oliverklee/task/gitignore

[TASK] Add a .gitignore.
Merge pull request #17 from oliverklee/cleanup/set-from-constructor

[CLEANUP] Use the existing setters in the constructor.
Merge pull request #18 from oliverklee/feature/unit-tests

[FEATURE] Add some basic unit tests.
Merge pull request #19 from oliverklee/task/usage

[TASK] Enhance the README.md.
Merge pull request #20 from oliverklee/cleanup/braces

[CLEANUP] Clean up the spacing around curly braces.