add log mapper
added log ticker
added log tick formatter
added log axis
added log items to main.coffee
added log items to base.coffee
added mapper_type attribute
added continuous axis and log axis to objects.py
added way to ask for log mapper type
added log plot example
Fixed example name and relocation to plotting/file because the example use the high level API.
Merge pull request #773 from ContinuumIO/feature/logaxis

Log Axes support