Add session_info function

This overrides the `utils:::print.sessionInfo` function so that it prints out a package's `GithubSHA1` value, if present. Output looks like this:

```
> library(devtools)
> library(ggvis)
> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-apple-darwin13.1.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] ggvis_0.3.0.99(70aef48) devtools_1.5.0.99      

loaded via a namespace (and not attached):
 [1] assertthat_0.1             bitops_1.0-6               caTools_1.17              
 [4] digest_0.6.4               dplyr_0.2                  evaluate_0.5.5            
 [7] htmltools_0.2.4            httpuv_1.3.0               httr_0.3                  
[10] magrittr_1.0.1             memoise_0.2.1              parallel_3.1.1            
[13] Rcpp_0.11.2                RCurl_1.95-4.1             RJSONIO_1.2-0.2           
[16] shiny_0.10.0.9002(89dc132) stringr_0.6.2              tools_3.1.1               
[19] whisker_0.3-2              xtable_1.7-3              
```

It could also be invoked without loading devtools, with `devtools:::print.sessionInfo(sessionInfo())`.

@hadley what do you think about this approach, of overriding the print method?

I just realized that one drawback of putting this function in devtools is that simply loading devtools also results in loading a lot of other packages, making it harder to get a clear view of packages loaded when diagnosing some other issue. For example, with a new R session:

```
> devtools:::print.sessionInfo(sessionInfo())
R version 3.1.1 (2014-07-10)
Platform: x86_64-apple-darwin13.1.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
 [1] devtools_1.5.0.99 digest_0.6.4      evaluate_0.5.5    httr_0.3         
 [5] memoise_0.2.1     parallel_3.1.1    RCurl_1.95-4.1    stringr_0.6.2    
 [9] tools_3.1.1       whisker_0.3-2    
```