Some updates to the default DESCRIPTION file

I started with a clean slate on this pull request to address the issues discussed in #411. For reference, here they are again:

1) http://stackoverflow.com/questions/17662713/devtools-description-file
2) https://gist.github.com/mbannert/8403801

I made the default `DESCRIPTION` file created by devtools more similar to the default `DESCRIPTION` file created by RStudio (when creating a new package). `Authors@R`, `License`, and `Suggests` still defer to `getOption()`.

Here's the result of my changes:

```
> library(devtools)
> create("testPackage")
Creating package testPackage in .
No DESCRIPTION found. Creating with values:

Package: testPackage
Type: Package
Title: What the package does (short line)
Version: 0.1
Date: 2014-02-18
Authors@R: "First Last <first.last@example.com> [aut, cre]"
Description: More about what it does (maybe more than one line)
Depends: R (>= 3.0.2)
License: What license is it under?
LazyData: true
Adding Rstudio project file to testPackage
```

And immediately calling `document()`, which caused an error with the previous default value for `Authors@R`:

```
> document("testPackage")
Updating testPackage documentation
Loading testPackage
Updating namespace directives
Writing testPackage.Rd
> 
```

Finally, after setting some options (temporarily):

```
> options(devtools.desc.author = '"Nick Carchedi <nick.carchedi@example.com> [aut, cre]"', devtools.desc.license = "GPL (>= 2)")
> create("testPackage2")
Creating package testPackage2 in .
No DESCRIPTION found. Creating with values:

Package: testPackage2
Type: Package
Title: What the package does (short line)
Version: 0.1
Date: 2014-02-18
Authors@R: "Nick Carchedi <nick.carchedi@example.com> [aut, cre]"
Description: More about what it does (maybe more than one line)
Depends: R (>= 3.0.2)
License: GPL (>= 2)
LazyData: true
Adding Rstudio project file to testPackage2
>
```