added commonancestor method to call geogit merge-base command

Includes some cleaning and improving of docstrings in repo class
modified progress listener to adapt to new syntax in py4j connector
changed how init params are passed to geogit (fixes #26)
added geomfieldname method to feature class
fixed documentation (fixes #18)
added overwrite option to exportpg method (fixes #19)
fixed #17
added --defaulttype switch to exportshp, to allow exporting even when there are mixed feature types
fixed "continue" method, so it only applies to rebase not to merge command
fixed broken tests
removed debug line