remove :environment from the data transmitted

capturing locals interferes with the benefits we get from locals
clearing and was always of questionable utility
just pass in the prev-context, not the whole environment
define/use resolve-local to reference previous &throw-context
add test for resolve-local
use a size 1 boolean array as a mutable cell

an atom seems overkill here, this is a one time transition from false to
true and the location is only ever going to be available within one
thread.
extend the notion of wrapper to all IExceptionInfo objects
tests for raw ex-info compatibility
update README and doc strings to cover ex-info compatibility
doc tweak
Merge pull request #45 from scgilardi/use-boolean-array-for-threwQ

use a size 1 boolean array as a mutable cell
Merge pull request #44 from scgilardi/stop-capturing-locals

Stop capturing locals
remove :environment reference from README. overlooked in #44
fix doc re: cause being passed to ex-info rather than captured
Merge branch 'issue-35'

Conflicts:
	README.md
	src/slingshot/slingshot.clj
remove comment about capturing &env, no longer true
improve docs for the else clause

since it doesn't affect the return value, like code in a finally clause,
it is only useful for side effects
spiff up the markdown
use `when` for side effects, `if` for expressions
regularize "message, cause, stack-trace" args
provide `with-cause` to allow specifying the cause explicitly

previously the cause was automatically captured within a try+ catch
clause or nil otherwise.

This could also be accomplished by allowing more flexibility in the
throw+ arguments, but that's quite a bit more complicated to code.