Version bump: 0.4
Set environment variable `HASKELL_PACKAGE_SANDBOX` on activation

Some external tools (e.g. the `doctest` package) use this variable to
locate the correct GHC package database to use when running in a
sandboxed environment.

Closes #20
Unset `HASKELL_PACKAGE_SANDBOX` variable on deactivation
Add plumbing for URLs and versions. Fix warnings.
Use Url instead of URL for constructor.
Modify GHC option to allow versions and URLs.
Fix a minor mistake
Enable automatic downloading of a binary distribution of GHC when creating a new sandbox

Closes #22

Merge branch 'ghc-urls'
Split overlong lines
Update to work with default ".hsenv" directory
Fix Emacs support

This has been broken since the reintroduction of single-sandbox mode in
c5922debe858d4b2c259161ddf7aecdb3bafb5df.

Merge remote-tracking branch 'github/nominolo/master'
Refactor for readability
Fix comment
Further fixes for broken Emacs support

Merge remote-tracking branch 'github/nominolo/master'
update emacs support for GHC 7.6
fix search for hsenv directories
use appropriate option names for older GHC versions
tidy up directory search; remove trailing whitespace
use absolute paths in directory search
support several package-dbs