Updated plurals.py to match Mozilla and CLDR references

Updated plurals.py to match Mozilla and CLDR references

This commit updates the plural rules, and in some cases the plural
number, for languages based on review of
https://developer.mozilla.org/en-US/docs/Mozilla/Localization/Localization_and_Plurals,
and
http://www.unicode.org/cldr/charts/29/supplemental/language_plural_rules.html.

I changed the interface (either the number of plurals, or the meaning
of each index), for the following languages:
• Arabic (ar): numplurals same, semantics differ per Mozilla's "Plural rule #12". 3 has been made more accurate, 4 has been switched with 5, and the meaning of both has been refined to match the rule.
• Belarusian (ar): numplurals updated to 3, using Mozilla's "Plural
rule #7". Previously the default, where anything except one is plural.
• Breton (br): numplurals updated to 6, using Mozilla's "Plural rule
• Bosnian (bs): numplurals updated to 3, using Mozilla's "Plural rule
• Irish Gaelic (ga): numplurals update from 3 to 5, using Mozilla's
"Plural rule #11". Cases have been added for n between 3 and 6, and n
between 7 and 10.
• Icelandic (is): same numplurals (2), semantics updated using
Mozilla's "Plural rule #15". 0 now means that n ends in 1, and is not
11, instead of just the number 1.
• Maltese (mt): same numplurals, same rule. Fixed rule, index 1 now
includes numbers ending in 1 (except the number 1)

I'm ignoring these two rules listed on the Mozilla resource
• Hungarian - listed as `(n != 1)`. It is not required to pluralize a
noun if a qualitative or quantitative amount precedes it
http://www.hungarianreference.com/plurals.aspx.
• Vietnamese - listed as `(n != 1)`. It only seems to have one plural
according to this lesson:
http://yourvietnamese.com/vietnamese-grammar/nouns-in-vietnamese/.

There are some additional languages listed in the Mozilla reference
that are not in the plurals document, and could be added:
• Lower Sorbian (dsb) (4, '(n%100==1 ? 0 : n%100==2 ? 1 : n%100==3 ||
n%100==4 ? 2 : 3)')
• Upper Sorbian (hsb) (4, '(n%100==1 ? 0 : n%100==2 ? 1 : n%100==3 ||
n%100==4 ? 2 : 3)')
• Scottish Gaelic (gd)  (4, '(n==1 || n==11 ? 0 : n==2 || n==12 ? 1 :
(n>=3 && n<=10) || (n>=13 && n<=19) ? 2 : 3)')
• Faroese (fo) (2, '(n != 1)')
• Frisian (fy) (2, '(n != 1)')
• Macedonian (mk) (3, '(n%10==1 ? 0 : n%10==2 ? 1 : 2)')

Closes https://github.com/python-babel/babel/issues/430