Ability to more flexibly define the path to a text file

This allows a user to use tilde expansion or simply the file name if the text file is in the user working directory.
I think path expansion is a good idea, but I am not sure about pre-pending getwd().

In distributed settings the path is usually a HDFS file or S3 file and looks like hdfs://<master>:9000/file or s3n://<bucket>/file etc. (And checking for all these prefixes might be painful).

BTW doesn't R automatically figure out relative paths ? For example I've used `textFile(sc, "./README.md")` before and it used to work.