Multiple improvements to the library.

[Jaesin]

This pull request has the folowing:

• Fixes the docblocks.
• Adds some basic tests.
• Adds an interface for the client.
• Adds Travis CI which is free for open source projects.
• Adds a getFields() (Describe lead) call,

Let me know if you need help maintaining this library.

[Jaesin]

Let me know if there are any adjustments needed for this pull request.

Test results are here: https://travis-ci.org/Jaesin/marketo-rest-api

[Jaesin]

@dchesterton I converted the API integration tests to use the built in guzzle server instead. Let me know how we can work together on getting this pull request merged.

It would be great if you could enable travis ci for this repo. Test results here: https://travis-ci.org/Jaesin/marketo-rest-api