Updates for SilverStripe 4 compatibility

[robbieaverill]

Resolves #12

This pull request updates the module for SilverStripe 4 compatibility.

Pre-requisites

• [x] Release a 1.0.0 stable tag of the current master (up to you, but makes sense - the current composer.json is aliasing these changes as 2.0.x-dev expecting that you will release the current version as 1.0.0, but if you don't, the alias should be updated to 1.0.x-dev instead)
• [x] Branch current master branch off to 1 or similar for maintaining the current 3.x compatible version going forward (see above)

Changes

• Rename the "code" directory to "src" for PSR-4 compatibility
• Add BluehouseGroup\Pardot namespace to all classes (except the Pardot class in the thirdparty dir!)
• Implement core framework namespacing throughout the classes
• Update the readme and configuration files
• Add mapping file for the silverstripe-upgrader utility - this helps people using your module to upgrade their code to your new namespaces
• Apply some PSR-2 linting and minor formatting updates, fix some doc block formatting
• Replace the RestfulService implementation with Guzzle - SS4 removes RestfulService completely
• Apply Bootstrap 4 classes and wrappers to the PardotModalForm template buttons and inputs
• Added the BH_PARDOT_DIR constant to represent the installation folder instead of hard-coding it

Notes/to-do

• The default inline form grouping at time of writing for the SS4 CMS doesn't look perfect for the attribute inputs. I decided not to add custom CSS for this since you don't have any at the moment, but noting it here:

• The "A" and "B" are not inline in that screenshot either. Could probably be some sort of Bootstrap inline label or something that could be applied to it.

• I haven't updated the screenshots

• I've tested this against 4.0.0-alpha4.

[andrewvt]

@robbieaverill - I believe I have created the release and the branch as you requested, both great suggestions. Please let me know if the pre-requisites have been met now and I will go ahead and do the merge. Thanks!

[robbieaverill]

@andrewvt yeah nice! Good from me now.