change in the behaviour of createLocalRepo - the force argument

[pbiecek]

Currently, the force parameter in the createLocalRepo() function forces to create a new directory if one does not exist. But by default it's TRUE and I do not see why anyone would set it to FALSE.
As in #318 it is more natural to check the opposite.
Proposed behaviour:

1. createLocalRepo() creates dir is one does not exist
2. if directory exist and backpack.db is there, then force=TRUE is required to reinitialize the repository
3. by default force=FALSE

[pbiecek]

fixed with 03323a9
As soon as Travis finish all tests this version will be submitted to CRAN

[pbiecek]

fixed and warning is added to https://github.com/pbiecek/archivist/blob/master/NEWS.md