Fix panics and/or EOF errors during GetPrinters request

fix #30

Fix panics and/or EOF errors during GetPrinters request

this release should fix #30

v1.6.0

• With this release the server communication is moved to a separate interface. Currently there is an implementation for http and the local Unix socket of the CUPS server. The IPP client still uses the http implementation by default. (@dennisoehme)
• the get job attribute function now returns the job attributes (@the-goodies)

v1.5.0

Breaking changes:

• resolution struct: use int32 insteat of int

Other Changes:

• added limit and number of document attributes to tag mapping
• fixed misspelled printer state reasons attributes (added missing s at the end)
• fixed some spelling issues

v1.4.3

removed status code check in response decoder which could lead to a panic if the server doen't return a status message

v1.4.2

• check ipp reponse for errors
• moved print test page funtion to cups client because the test page has a cups specific mime type
• added comments for godoc
• added link to godoc and go report in readme

v1.4.1

Fixed eof handling in response and request decoder

Version v1.4.0

• breaking: removed job and operation prefix from attributes
• breaking: use built-in data types for constants
• fixed some unexportet constants
• some fixed for reponse encoding

Version 1.3.0

• added request decoder method
• added response encoder method
• better EOF handling in reponse and request decoder
• decode response from http body instead a temporary buffer

Version 1.2.0

• changed the get jobs method to fetch jobs from printer, classes or both
• the response decoder now need a writer for additional response data, for example if you fetch documents from the server
• added the job state filter const "all"