[unset-GHC_MODE-env-var] unsets the PACKAGE_DB_FOR_GHC_MOD

- Not sure if this is causing anyone problems, but it looked like an
  oversight.
s/MyFoo/HsenvFoo/

Not that the previous naming convention was outright *bad*, but this is
a bit more idiomatic.

Closes #30
Fixes #23
Use `throwError` to signal GHC tarball download errors
Use the `http-streams` package to download GHC tarballs instead of an external `curl` binary

Merge branch 'issue-23'
Remove unused module `Util.Tar`

This was apparently a never-completed internal tarball extraction
implementation. In reality, hsenv just shells out to the system's `tar`
binary.
Formatting
Clarify the hsenv directory name in the usage message
Don't auto-wrap the usage message "outro"

This screws up explicit formatting.
Add a usage blurb about activating and deactivating the sandbox

Closes #35
Tentative doc update
Fixed char stripping of md files
updated README to support code correctly
Final commit before first PR
Various minor tweaks to README content on top of adinapoli's work
Shell snippet formatting

Adds syntax highlighting (via GitHub Markdown or Pandoc) and removes the
unnecessary leading `>`s in code blocks.
Documentation modernization and cleanup

Merge branch 'updated-docs'