Reorganized Classes

Renamed SwissInpaymentSlip to SwissInpaymentSlipData
Renamed SwissInpaymentSlipPdf to SwissInpaymentSlip
Removed FPDF specific code from SwissInpaymentSlipPdf into new SwissInpaymentSlipFpdf

Updated two example files
Created an abstract base class for SwissInpaymentSlipFpdf named SwissInpaymentSlipPdf.
Hopefully this will allow to use other pdf engines than FPDF
Removed a left over function in SwissInpaymentSlipPdf
Fixed several bugs
Improved code flexibility
Changed private members and methods to protected because this helps to extend this class mor easily
Improved performance by utilizing cached hex2rgb responses and conditional coloring
Added TCPDF to composer.json
Created two examples to test the classes with TCPDF