Simplify/Unify handling of resources in HTMLFileSession
Allow to use development frontend files in HTMLFileSession

Possible configurations:

session = HTMLFileSession("some.html")

session.save(resources='inline')"
session.save(resources='relative', rootdir='.')"
session.save(resources='relative-dev', rootdir='.')"
session.save(resources='absolute')"
session.save(resources='absolute-dev')"
Better error handling in HTMLFileSession.get_resources()
Set HTMLFileSession.rootdir to current directory
Use _AttrDict instead of dict for plotting._config

This allows for _config.attr access along _config["attr"]. Also,
_config.attr = value requires attr to be predefined on _config
(e.g. in _set_config()) to disallow misspelling of attributes.
Allow to override configuration options with BOKEH_* variables

BOKEH_FILE_RESOURCES="absolute-dev" python bollinger.py
Better error handling in _AttrDict
Handle BOKEH_{RESOURCES,ROOTDIR} in HTMLFileSession.get_resources()
Use os.path.realpath() on cygwin in HTMLFileSession.static_path()

This allows to use 'relative*' along 'inline' resources. realpath()
is required to resolve symlinks.
Update docstrings and documentation (quickstart.rst)
A few minor fixes to documentation (spelling and syntax)
Reword "Generate static HTML files" section
Add contents to "Sessions" section of dev_guide.rst
Fix syntax in dev_guide.rst
Add support for HTMLFileSession.save(resources="cdn")
Use realpath() instead of abspath() in versioneer.py

This is required to make bokeh.__version__ work with symlinks.
Use bokeh.__version__ to determine HTMLFileSession._cdn_version
Merge pull request #419 from mattpap/resources

Improve handling of static resources in HTMLFileSession