LIB-47: I18n of error messages

http://tickets.musicbrainz.org/browse/LIB-47

This is work in progress. A bit of testing code was included so a string is printed on discid_new() invocation. That string is translated using gettext. (needs more work).

Building itself works fine on Linux with cmake and autotools.
Autotools already includes near full gettext support. implementing --disable-nls and a fallback when libintl.h is not found is still missing. .pot/.po are updated with "make update-po" in po/ or with make dist.

CMake builds don't have any support for .mo generation/installation, but the binary is able to use these (when installed through other means). Shouldn't be too difficult to create targets manually though.

Not testing on other platforms was done yet.