ENH: better support for DEPS.

- remove empty and comment line from input
- extract the variables (Vars)
- replace Var() in the file
- extract the external DEPS (From)
- download external DEPS file (File)
- extract external deps from ext DEPS file

TODO:
- replace the external deps in the file
- run eval on all values of key/val pairs
- distinguish between git and svn (not really needed here, but ...)
- generate cmake file with wrapping around svn calls