Added a text argument to read.px to allow parsing a PX file from text#11

The native `read.table` function, of which `read.csv` etc. are based, offer a `text` parameter where you can submit the file as text. This is convenient when e.g. reading a downloaded file directly. By adding this argument, `read.px` implements an interface closer to the ones of built in file readers in R. With this change, reading a file from the web can be done like this: ```r req <- GET("http://www.example.com/file.px") px_raw <- content(req, "text") px <- read.px(text = px_raw) ```