Massively overhauled the entire "framework"

* mg is back to a function (sorry @philipdexter). I really
  wanted the snap functionality to be included in core mg
  instead of an external .sh file.
* mg subcommands (eg add, rm) are now defined as functions
  prefixed with mg- (eg mg-add, mg-rm). This lets users write
  their own mg extensions by just defining a function. NOTE:
  mg extensions are *not independent* of mg! They must be
  called by mg or they'll probably break the ~/.multigit
* Code has been separated into 4 separate files:
  * multigit.sh: just sources the rest of the code
  * main.sh: primary mg function / framework
  * builtins.sh: included mg extensions (add, rm, list, go, help)
  * util.sh: utility functions (dirExists, gitExists)
* Bugs and missing features may be abundant. Look for FIXMEs
  to find out what needs to be fixed still.
Clean up util warn functions
Fix mg help
Improve extensibility for commands that default to reading input
Fix hardcoded function name in recursive code
Change namespace from mg to multigit

Fix #7
/facepalm
Fix multigit.sh sourcing issue
Update README
Update README.md
Merge pull request #8 from scizzorz/turbo

Massively overhauled the entire "framework"