introducing the core module

only one import for users
Central PyiUpdater object to rule them all
updated tests with new PyiUpdaterConfig
updated TUI with new pyiupdater object
Add subcommands to cli

Also fixed failing tests
switch to py.test
updated travis with py.test
updated travis script
removed duplicate default values
updated travis requirements
removed distributed test option
can't figure out why travis tests are failing
added missing requirement
switched from travis to circleci
updated circle yml
updated for tox usage
...
added coveralls config
moved imports to fix import error
moved version info to separate module