Change default IP address lookup URI

icanhazip.com has two other domains for fine-tuned IP lookup,
ipv4.icanhazip.com will always return the IPv4 address, and
ipv6.icanhazip.com will always return the IPv6 address (the latter
appears to only work if you actually have an IPv6 address).

Details at http://rackerhacker.com/icanhazip-com-faq/.

Closes #68.
Make VCS guess() function respect .git files

Closes #67.
Raise TypeError only if it is segment type that has problems

It raised TypeError also when module did not have required function
Add Gentoo live ebuild

Still needed to push this ebuild to some overlay or main tree; otherwise
it requires more effort to use.

Users need to use

    eselect fontconfig enable 10-powerline-symbols.conf

to actually use the fontconfig file (should be added to the docs).
Ebuild itself installs the file, but doesn't enable it.
Cleanup weather condition codes
Addd “default_module” theme key support

Refs #3.
Move terminal and tmux segments into common module

This should ideally be renamed to something else since it's not strictly
an extension. A better module naming scheme could be something like:

	powerline.segments.ext.{extension}
	powerline.segments.common

This would move all segments out of their respective extension
directory, and might be a bit confusing. The ext directories may also
need some changes to make this work properly.