Makefile: add a "dist" target

The dist target in the Makefile will take care of generating the manual
page, and also create an i3blocks-version.tar.gz gzip'ed tarball.

[vd: use Git to list files]

Closes #57
add incremental log level

Add a cumulative -v option to control the display of log messages. By
default, error messages are only displayed on stderr. With -v, block
update failures are also displayed within the bar. -vv enables debug
messages on stderr.
contrib: add new temperature script

This script uses the raw output to support any sensor chip.

[vd: squash and move to contrib]

Closes #30
scripts: (cpu_usage) use default error message

die() already provide a very similar default message.
readme: notice the ruby-ronn dependency

This tool is needed to manually generate the man page.

Closes #56
config: document the default config file

Add the list of valid properties in the default config file, as well as
a link to the manual page.
Makefile: fix VERSION test

Because I removed the double quotes of VERSION in Jason's d686490
commit, make is invoked with -DVERSION=\"\" outside a Git repository.

Reported-by: Jason Pleau <jason@jpleau.ca>