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.
Thanks Dan -- I'm still having trouble reproducing this though. I ran the following commands (listed below) on the development VM today and it seemed to work fine. (SparkR-pkg was cloned from github.com/shivaram/SparkR-pkg which is synced to a commit on Dec 22 FWIW).

Could you check the value of `getwd()` in R when you see the error ? I am wondering if R is somehow getting a wrong working directory.

```
[cloudera@quickstart ~]$ cd SparkR-pkg/
[cloudera@quickstart SparkR-pkg]$ R
> library(SparkR, lib.loc="./lib")
Loading required package: rJava
[SparkR] Initializing with classpath /home/cloudera/SparkR-pkg/lib/SparkR/sparkr-assembly-0.1.jar

> sc <- sparkR.init(master="local")
14/12/30 17:53:15 WARN Utils: Your hostname, quickstart.cloudera resolves to a loopback address: 127.0.0.1; using 172.16.123.130 instead (on interface eth1)
14/12/30 17:53:15 WARN Utils: Set SPARK_LOCAL_IP if you need to bind to another address
14/12/30 17:53:15 INFO Slf4jLogger: Slf4jLogger started
14/12/30 17:53:17 WARN NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
> 
> textFile <- textFile(sc, "README.md")
> 
> linesWithSpark <- filterRDD(textFile, function(line){ grepl("Spark", line)})
> 
> count(linesWithSpark)
14/12/30 17:53:18 INFO FileInputFormat: Total input paths to process : 1
[1] 37
```