Use standard value for hreflang tag

The hreflang needs to be in form language-country, not language_country.
make sure we don't redirect non-existent pages
Reduce size of flags

The PageSpeed tool of Google Webmaster tools signals that
"a lossless compression can reduce the file size to reduce page load time"

Let's do that: I reduced the size of the flags with this command line:
pngcrush -brute -force -reduce

The size of the images lowered from 327.367 to 233.841 bytes (28% gain).
Merge pull request #53 from mlocati/reduce-flags-filesize

Reduce size of flags
updating changelog
Merge pull request #51 from Remo/fix-home-redirection

make sure we don't redirect non-existent pages
Merge pull request #50 from mlocati/fix-hreflang-attribute

Use standard value for hreflang tag