`_transform` doesn't work when a NMEA sentence comes in across multiple lines

[fabdrol]

The _transform method in lib/index.js doesn't work when NMEA sentences come in across multiple lines. Example: I have a GNS 1000 bluetooth GPS receiver at home, which emits NMEA sentences. However, sometimes a sentence comes in across multiple lines. The Transform stream should recognise this by detecting the beginning of a sentence ($) and the end *xx of a sentence.

I'll take a look at this tonight.