StringConstants: Add important directories
BearCollector: Add default for bear_dirs
Section: Add comment

This explains why the iterator implementation is no bug.
Section: Spacing corrections
Section: Add copy()
Section: Add update()
BearCollector: Allow recursive directories
DefaultArgParser: Replace analyzers by bears
BearCollector: Add from_section classmethod
BearRunner: Better variable name
coalib: Add default_coafile
CliParser: Use right default args for reparse

This was missed during review of 4fd258ac.
settings: Add SectionManager

This adds a Manager object which takes care of all the section stuff,
namely:

- Reading all settings in sections from
    - Default config
    - CLI
    - Configuration file
- Collecting all the bears
- Filling up all needed settings
- Write back the new sections to the configuration file if needed
- Give all information back to caller
SectionManager: Add test