Fix names for version variables
Implement YCM_MAINTAINER_MODE option

If this option is enabled, YCM downloads all the files for all the
versions. It allows to create source packages that do not need a network
connection to be built.

YCM now always checks if the file exists in the download directory
before actually downloading it.

Fixes #58
YCMConfig: Print a "found" message if not quiet
YCMInternal: Print a message when using previously downloaded files"
Use quotation marks around filenames
Avoid multiple downloads during bootstrap unit tests
Merge branch 'maintainer_mode'