null PathEffect is ok

Bypassing setPathEffect when it's null results in a PathEffect from
a sibling line bleeding over. Make sure to set PathEffect to null
when this line does not want a PathEffect.
Merge pull request #34 from dgmltn/null-patheffect

Null PathEffect is ok
- Fixed #35
- Created useful constructors for formatters
Merge pull request #36 from douglasjunior/master

Fixed #35 and created useful constructors
Introduced small fix for #35 for situation when user set decimalDigitsNumer=0
Fixed broken formatters
Update java docs for ValueFormatterHelper