optimize execFunctionDeps loading in worker.R

using `rawConnection` to keep the `load` in memory;
see http://stackoverflow.com/a/23597894/4134338
[SPARKR-147] Support multiple directories as input to textFile.
further optimize using rawConnection

update `getDependencies`
Merge pull request #126 from sun-rui/SPARKR-147

[SPARKR-147] Support multiple directories as input to textFile.
remove memCompress
switch the order of packages and function deps
[SPARKR-164] Temporary files used by SparkR accumulat as time goes on.
Merge pull request #127 from sun-rui/SPARKR-164

[SPARKR-164] Temporary files used by SparkR accumulat as time goes on.
Merge pull request #125 from 7c00/rawcon

optimize execFunctionDeps loading in worker.R