Use explicit MathContext when truncation possible.

In Scala 2.10, many of the BigDecimal constructors will truncate the
provided number to the "default" MathContext (DECIMAL64). This fix
simply ensures that everywhere we use a BigDecimal constructor that
would truncate the input if it was large enough, we provide an explicit
MathContext.UNLIMITED so it won't.
Remove leading '+' when parsing BigInts.

I guess +1234 is illegal in Java 6, but not in Java 7. To be safe, we
always remove the leading '+' when parsing a BigInt.
Merge pull request #153 from tixxit/fix/bigdecimal-truncation-fix

Boom!