Switch to latest gettext API (>= 2.3)

gettext 2.3 changed tools name and call methods:
 - GetText::RGetText is now GetText::Tools::XGetText
 - GetText.rmsgfmt was killed by GetText::Tools::MsgFmt.run

Classes and file names have been updated accordingly.
Update Haml::MagicTranslations::XGetText interface for gettext 2.3

The parse method need only one argument. See commit 299aea17 in gettext.
Stop escaping '\n' when extracting markdown strings

This is now done at gettext level. See GetText::Tools::POEntry#format_message
and callers.
Document that strings extraction requires gettext >= 2.3
Remove extra whitespaces
Version bump to 0.5.0
Regenerate gemspec for version 0.5.0