fixe cache issue.

Won't render otherwise

the two path joined were both absolute. Now, use relative "base" isntead of full_base to build the base_cache_dir
Merge pull request #89 from bhesmans/fixCache

fixe cache issue.
Merge pull request #88 from metaswirl/master

First pull request :)
add shortcuts to common classes/functions
simplified access to update http
search for edges based on src/dst string ids
merge ospf areas back into general function
white body for printing
inc ver
highlights for nodes and edges
example ipython notebook
update workbook
more notebook updates
move to gist
add tutorial
update tutorial
allow type casting in copy edge and node attribute functions
extend tutorial examples
bugfix: dont set if node not in graph
set default igp