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.
Sorry for the delay -- I finally got a chance to try it out today in a couple of settings (with the SparkR script and with RStudio).

In both cases I found that current working directory worked directly without any need for adding `getwd`. For example, these were the commands I used in RStudio
```
> library(SparkR)
> sc <- sparkR.init()
> rdd <- textFile(sc, "README.md")
14/12/23 21:42:05 INFO FileInputFormat: Total input paths to process : 1
> count(rdd)
[1] 128
```

Just for reference the working directory contents were
```
> list.files(getwd())
[1] "R"         "README.md"
```
Would you happen to have any logs / a way to reproduce the problem you saw with quick start ? 

BTW I think we should anyways commit the `path.expand` change as it is good to handle things like `~` in the path.