Release 0.19.0

Another maintenance release with some small additions.
The main work is going into 1.0 at #558 right now.

This release contains the following additions:

• Checks for the right argument number in contract functions thanks to @cdetrio
• Fix for "filterCreationErrorCallback is not a function" thanks to @obale
• Export padLeft and padRight as functions on the web3 object thanks to @nanexcool
• added web3.eth.signTransaction() thanks to @denisgranha
• adding personal.importRawKey, personal.sign and personal.ecRecover thanks to @peculiarity

Also thanks goes to @peculiarity and @zmitton for smaller fixes.