Add support for Serbian (Latin)

[fatkaratekid]

Changes proposed in this pull request:

Add support for Serbian (Latin)
Adds support for local currency RSD, EUR and RUB

Status

• READY
• HOLD
• WIP (Work-In-Progress)

How to verify this change

Just run the tests.
I included tests for RUB, EUR and RSD.
Tried to cover all the possible case combinations (in grammar sense) and plural form changes.

Additional notes

The ordinal numbers have not been implemented yet.

[coveralls]

Pull Request Test Coverage Report for Build 483

• 66 of 70 (94.29%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 89.552%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
num2words/lang_SR.py	65	69	94.2%

Totals
Change from base Build 478:	0.03%
Covered Lines:	660
Relevant Lines:	737

---

💛 - Coveralls

[shulcsm]

Could you refactor base into something like:

    def _currency_units_verbose(self, number, currency):
        return self.to_cardinal(number)

    def _currency_fractions_verbose(self, number, currency):
        return self.to_cardinal(number)

to_currency:

        unit_str = self._currency_units_verbose(left, currency)
        fraction_str = self._currency_fractions_verbose(right, currency) \
            if cents else "%02d" % right

        return u'%s%s %s%s %s %s' % (
            minus_str,
            unit_str,
            self.pluralize(left, cr1),
            seperator,
            fraction_str,
            self.pluralize(right, cr2)
        )

So you don't have to overload whole thing?

[erozqba]

Thanks for taking the time to make this contribution!
Could you please also update the README.rst to include lang_SR?

[coveralls]

Pull Request Test Coverage Report for Build 535

• 66 of 70 (94.29%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 89.805%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
num2words/lang_SR.py	65	69	94.2%

Totals
Change from base Build 531:	0.1%
Covered Lines:	2537
Relevant Lines:	2825

---

💛 - Coveralls