Better Greek transliteration

The way Drupal transliterates Greek characters was not aligned with common practise by modern Greeks.

Minor changes applied. Specifically, now:

"υ" is turned to y instead of u.
"φ" is turned to f instead of ph.
"η" is turned to i instead of e.
"χ" is turned to h instead of kh.

For example, "η" was turned to e instead of i. So, the word Δοκιμή (test) was transliterated to Dokime, while everyone would expect to get to Dokimi.

For existing websites, if you use pathauto to autogenerate paths, this change will have an impact on your URL's. After you save an existing node, it will generate a new path according to these new transliterations. So, make sure that for these URL's, you either set a non-auto generated URL, or you set a redirect from the old URL to the new URL.