IP filter for bitbucket as suggested by @edwardmp

and supported by documentation at
https://confluence.atlassian.com/display/BITBUCKET/POST+Service+Management
code documentation updates
moved driver files
updated namespace and files
payload commit implementation
payload class and interface implementation
gitignore updated
bootstrap self-initialize
composer installation for symfony/process
renamed phpunit file
updated bootstrapper file to use composer autoload
updated test bootstrap
updated example hook to use self-initializing bootstrap
test updates
updated composer files for Deployer namespace loading
payload implementations
removed accidentally added file
updated Deployers to use Payload
updated tests
updated example