make PollHandler an interface

- introduce SimplePollHadler for services having same poll and final result
- introduce AbstractPollHandler and AbstractPollHandlerBase classes for services which need more poll customizations
- removed ugly instanceof operator in former PollHandler class
fix model service

- change updateProjectModel(Project, ModelDiff) to execute MAQL script from given ModelDiff with the least side-effects
- make getProjectModelDiff(Project, DiffRequest) private as it is not usable for the user
- add tests

Closes: #20
improve getProcessSource javadoc
add support for relative upload paths in DataStoreService

the link to uploads in Gdc, can be relative when the host is the same as gdcHost,
added fallback to complete the relative path with gdcHost uri
fix improper messages
Merge pull request #54 from martiner/poll

refactor PollHandler and fix model service
Merge pull request #23 from martiner/jmi-uploads

add support for relative upload paths in DataStoreService
Merge pull request #57 from martiner/processService

improve javadoc and messages in process api
rename Process to DataloadProcess
add ProcessType enum