BuildableCurrencyUnit should be Serializable

I believe that `BuildableCurrencyUnit` is intended to be `Serializable`
because `Money` is `Serializable` and has a field named currency which
is a `CurrencyUnit`. So in order for `Money` to be `Serializable` all
fields have to be `Serializable` as well. The other option would be to
mark `CurrencyUnit` as `Serializable`.

 * make `BuildableCurrencyUnit` `Serializable`
 * add test that ensures `BuildableCurrencyUnit` is `Serializable`
Merge pull request #48 from marschall/missing-serializable

BuildableCurrencyUnit made Serializable.
Delete Unused Classes

The classes
`org.javamoney.moneta.convert.DefaultMonetaryConversionsSingletonSpi`
and `org.javamoney.moneta.convert.TestMonetaryConversionSingletonSpi`
are not referenced anywhere and are not tests either. They can be
deleted and the number of tests stays the same and no tests fail.
Merge pull request #49 from marschall/delete-classes

Delete Unused Classes
Fix Compiler Warnings

The project contains several compiler warnings.

 * remove unused imports
 * remove unused methods
Merge pull request #50 from marschall/compiler-warnings

Fix Compiler Warnings
Used Math class in other locations within FastMoney.
Minor changes.