Mention `base64enc` dependency in install instructions

[geneorama]

The command install_github('ramnathv/rCharts@dev') fails unless you have the CRAN package base64enc installed.

> sessionInfo()
R version 3.1.2 (2014-10-31)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8       
 [4] LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C              
[10] LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] base64enc_0.1-2  geneorama_1.4    RUnit_0.4.28     data.table_1.9.4

loaded via a namespace (and not attached):
 [1] bitops_1.0-6   chron_2.3-45   devtools_1.7.0 httr_0.6.1     plyr_1.8.1    
 [6] Rcpp_0.11.4    RCurl_1.95-4.5 reshape2_1.4.1 stringr_0.6.2  tools_3.1.2  

[geneorama]

Also, rjson needs to be installed to run the first example.

I think there may be a problem with how the packages dependencies are defined, because normally the dependencies would be installed with the package.