[cli] Ported the clean command to the Core standard library.

Conflicts:
	cs3110-cli/staff/command/clean.ml
[cli] Remove duplicate postscript files.
[cli] Add new includes to corebuild.
[cli] Add the str package to corebuild imports.
[cli] Get clean command to comply with new interface.
[cli#49] Export additional functions.
[cli] Remove unused includes in corebuild.
[cli #49 #55] Add individual targets to clean command.
[cli #49 #55] Add cHARNESS_DIR alias to cOUTPUT_DIR.
[cli #49 #55] Simplify dispatch for clean command.
[cli #49 #55] Reword help messages.
[cli #49 #55] renamed Clean.build to Clean.compile
[cli #49 #55] abstracted clean targets readme
[cli #49 #55] clean default is compile
[cli #49 #55] removed _Util.ml files
[cli #44 #55] core pipeline, no more string parsing Sys.argv
[cli #44 #55] cut redundancy in clean commands
[cli #49] merging bkc clean command
[cli #49] using Core.Std.Unix
Merge pull request #65 from cs3110/cli-clean

Port clean command #bkc